I'm only 24, so sorry if I'm not the demographic you were looking for opinions from lol.

It doesn't seem like you're too excited to have a future (married or unmarried) with your girlfriend as it is. Sometimes "work it out" isn't a good answer because some things cannot simply be worked out. I was with my ex for 4 years and we had an ok relationship but I think we stuck together because it was comfortable, not because were very happy. That couldn't be worked out. Sometimes you spend a few years with someone and then realize hey maybe we don't want the same things, maybe this isn't what we thought it would be in the beginning.. And that's totally ok. I personally don't think relationships are ever a waste of anything. Growing with someone, whether it ends in a happy marriage or not, is not a waste and you gain experiences because of these relationships.

I don't think marriage will solve any issues you have now, and sticking to how you feel is not being selfish or unfair. Maybe this isn't the opinion you were looking for, but I think you have already kinda made up your mind.