A few of my close friends / family are still with their first gf/bf decades on, and thankfully I think they are all very happily married.
It seems some people are just very fortunate and meet someone they could spend their life with very early on. (This only works if you are both already pretty much who you're going to be, or you change and mature in the same way. There is a bigger risk of 'growing apart' when you get together very young).
The benefit of having several bf / gf before settling down is that you have something to compare with, rather than assuming that something is normal or to be expected just because your only bf does it.
The risk of being with your first partner forever is that you could be left with 'what if...?'
The (couple of) people I know who have regretted it, committed more than they should to their first bf because they felt pressure to settle down / get a ring / have a wedding. By the time they realised they'd made a mistake they were too far down the slope and then just continued being swept due to either a fear of changing things or (real or perceived) family expectations. It's sad as I think they may lay on the death bed and think 'what if?'. In one case, my friend's life would have been so different if she was single, or with someone more suited, and she is missing out on experiences for fear of rocking the boat.
It may be that your relationship has run its course. If it has there's no need to feel guilt. It just is what it is. But for both your sakes you need to get your head around ending things. Please don't look back with regret in your old age.