MMB, hopefully you are still reading through the responses that you get on here.
I'd like to tell you the experiences of my BF as they are very similar to yours. I will tell you what happened in her situation, it may help you.
My BF met her then DP when she was 15. She'd never been in a relationship before (so different to you in that case) but knew that she loved him. Again, it was slightly different in that he was her first and only sexual partner so it was exciting for her at the beginning. But like a lot of long-term relationships, the spark goes or changes.
After being together for 11 years, they got engaged and planned to marry the following year. They'd bought a house and a couple of cats and were settled. They had their friends and my BF would come out with me on Saturday nights and her DP would go out with his mates and we'd meet at the end of the night.
She has always been naturally flirty with men but it had never meant anything. She was constantly being told that they were an odd match. From the outside, their looks didn't seem compatible and she was told often (and sometimes by complete strangers
) that she was much too good for him and could do better. But I knew them well and she never thought that.
However, she met a woman through work and started spending a lot of time down at her house in the evenings. It then materialised that this woman had a son who my best friend had totally clicked with. She absolutely knew that she needed to keep away and hadn't even told me about him (it was only after it all fell apart that we found out).
On her hen-night (that I'd arranged), she kept on crying and no-one could understand why. But I knew that there was something terribly wrong.
Her parents had very little money and her Dad had just been diagnosed with Parkinsons, so I believed at the time that it was related to this.
Two weeks later they had a beautiful lavish and expensive wedding. After 13 years together, they'd finally tied the knot.
Eight months later, my BF left her DH. A month after that, we found out that she was with this other guy and had had feelings for him since before they married (although she swears to me that she never did anything with him while she was with her DP, but knew that her feelings were very strong).
5 years later, they are still together. Her Ex-DH was devestated and is unfortunately still single.
My BF is desperate to have a DC, but even though she is with her DP in relationship terms, he lives 100 miles away from her and only sees her at weekends. She has been giving him the same ultimatum for 5 years that he needs to move down, otherwise she will leave him and he still hasn't.
So, I guess the moral of this story is... As much as it may be really really hard, if you're not sure, don't go through with the wedding. Call it off sooner rather than later. BUT... If that's the route you go down, don't assume that things will be rosy with this other person. Have some time on your own before committing to anyone else.
My BF lost the respect of all of her other friends, her ExH's family and no-one has ever really accepted her DP. I've only met him twice in 5 years, even though I see her almost weekly. And even all this while later, she hasn't really got the commitment that she wants and at the age of 34, if she wants to have a child, is going to have to start a whole new relationship with someone else as he is not really bothered about having one, which takes time and the clock is ticking for her. (NB: I only had my first child at 34 so I know it's not unusual, but she needs to finish things with her DP first so that she can meet someone else and she's just not ready to do that yet).
One final thing though, she NEVER regretted her decision to leave her ExH, but just wished that she had done it before the wedding as she knows that she humiliated him by leaving so soon afterwards and would have spared her DParents the stress and hefty costs of the wedding.
Lots to think about... But when I read your post, it did ring of some similarities...