Let me tell you about the sort of friend Sam is. In Year 11, after I’d downed one too many warm beers in a lame effort to impress a shaggy-haired boy named Andre, Sam risked life and limb to sneak me back into her house, tiptoed into the kitchen for emergency provisions, and held my hair back as every drop of my six-pack chucked its way down the toilet. A year later, when my father died tragically, Sam held my hand so tight at the funeral that her fingernails left indentations in my palm. (Every year, she still marks the anniversary of that wretched day with white tulips and a phone call.)

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In university, when I failed to gain admission into a postgraduate program, it was Sam who glued my self-esteem back together, sending me email reminders about previous victories and recounting every nice thing she ever heard said about me. It was also Sam who marched into my flat six months later and demanded that I get my “arse off the lounge” and reapply – and who bought the first round of drinks when my acceptance letter arrived.

She is my favourite person on earth, and the one who I feel is most deserving of all of the happiness that life has to offer. So last June, when she rang me – I was third on the list behind her mother and someone I’m now jealous of – to tell me her brilliant boyfriend, Mike, had just proposed, the emotions I felt were pure and uncomplicated: joy, delight, elation. The woman I cherished was going to get the lifetime of love she deserved.

So why now, as the big day creeps closer and it emerges that this wedding is more than a brilliant excuse for a party, do I feel so strange about it?

There’s a voice in my head that I just can’t shake: this is it. This is the end of something. What you have with Sam won’t ever be the same again.

This is not about jealousy or disappointment with my own life. I’m happy, no question about it. I have meaningful work, wonderful friends and a genuine belief that when the time is right, my own “Mike” will walk into the bar. What it is, I think, is a dread that my beautiful, brilliant friend – and our beautiful, brilliant friendship – will change when Sam becomes somebody’s wife.

It happens. Every friend I’ve ever had who gets married has said as much.

“Oh, I really didn’t expect it to, but it does feel so ... different.”

“Better” is what they mean. Better than before, when you were all they had. And the real knife-in-the-back implication: better than what you’re left with, you poor, unmarried creature.

I’ve seen it happen to women less primed for partnership than Sam. Regardless of the independence they maintained before the wedding, once the ring goes on the finger, it’s like they get drunk on married. It makes stereotypes out of the most capable of women: “Oh, I’d love to come for drinks, but I can’t because the hubby is on his way home and needs his dinner.” Perfectly normal reasons not to hang out, but they still leave me without a partner in crime.

So it’s not really about that. I don’t mind that Mike takes the place of me at brunch or family dinners (well, I do, but a manageable amount). What stings is that he’s taken my place at the comfort table as well. When Sam has a miserable day at work, it’s Mike who hears about it. When she falls out with her mum, or feels paralysed with fear about her credit card debt.

For career prospects, Mike gets that call. He’s the sounding- board and source of comfort. And well he should be; he’s going to be her husband, for goodness sake. But while that makes me genuinely happy for her, it has also disturbed the careful balance of our friendship. Thing is, I used to be that person for Sam. If Mike takes over that role (and, let’s face it, it is inevitable that he will), where does that leave me?

If she doesn’t come to me with her stuff, how am I supposed to go to her with mine? She won’t need me anymore, but I’ll still need her. And that’s where the problem comes in.

I don’t think I’m alone in saying that as a single woman in my 30s, I’ve predicated a lot of my happiness on the fact that I’m not needy. I’ve worked hard to create a life that feels rich and exciting without the aid of a better half. (I don’t sob in a heap on my kitchen floor when I can’t reach the light fixture to change the globe – I just get a ladder.) But what I hadn’t realised is that, somehow, I’ve let Sam become my better half. Because we waited until our 30s to settle down, we took roles in each other’s lives that we may not have had we paired off in our 20s. She became my life keystone. And I hers. Until, by way of engagement ring, I suddenly wasn’t anymore. That’s Mike’s job now. In all his husband-to-be glory.

So, am I losing her? Why does it feel like I am?

Like most women in possession of half a dozen brain cells, I understand that I’m not really losing my best friend to marriage. I know that the blush of being a newlywed will wear off and that she’ll soon start coming to me again. Maybe not for everything that she used to, but probably for a few new things, too. After all, you can’t very well complain about your husband to your husband, can you?

I know that our relationship will change and make room for Mike and their eventual offspring, and all of this will be OK.

I get all of this. But I still feel weird about it. It’s like she’s moving on to some new stage in life and I’m lacking the pass to cross with her. She’ll be a wife. And with that title will come all sorts of twin sets and respect and responsibility. And if she’s all that, then what am I? What if all the stuff that has bonded us until now – our love of travel and karaoke and vodka, lime and soda – can no longer sustain us? What if it gets swallowed up by wifedom?

Go ahead and tell me it sounds ridiculous. I know it does. But for the past 31 years, Sam and I have been the primary witnesses to each other’s lives. We’ve had other friends and our families, of course – it’s not like we’re maladjusted – but we were each other’s rocks. Now she’s got a new rock, and I’m in a hard place.

In some weird way, all of the romantic, everlasting love that Mike gives Sam trivialises what we feel for each other. Friendship is the only sort of love that has no standing in society. It doesn’t grant you access to a hospital room.

It doesn’t make missing persons appeals on TV. It’s not an emergency contact number. As far as onlookers are concerned, as “just” a friend you’re on the sidelines. And that’s just not fair. I was there first. What about us? Is “us” now less important than “them”?

And then it dawns on me: not less important, but different.

So here’s the conclusion I’ve reached: friendship, just like the rest of life, is a constantly changing thing. Although the essence of what connects you may remain the same, the relationship will continue to reinvent and remould itself.

And you can’t freeze it in time at a supposed heyday, clinging to the shards like an old woman mourning the lost beauty of her youth. You can only embrace it, love it and move with it, celebrating each metaphorical laughter line along the way.

That’s the way not to be left behind as your friend moves on. Besides, it’s not just you who’s scared.

“When I get married, will you assume I’m always busy with Mike and stop inviting me to places?” Sam asked me this over coffee, after an afternoon spent shopping for wedding dresses.

I told her of course not, but the fact is, I don’t know. It could happen. In fact, it probably should happen.

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I can only promise Sam what I’ve always promised her – that I’ll always be there for her if she needs me, that while our friendship will change as a result of external forces in our lives, it will never disappear because of them.

She may have become someone’s wife, but she’s still my best friend – and even if she and I are the only two who understand how fundamental that is...well, that’s enough.