First of all, don't propose at 18. You can, you are allowed to, it's romantic and passionate, but you shouldn't. Pace yourself, experiment life, I don't want to sound like an old fart, but things are not how you see them at 18. Please take it slow, for your own good, longterm.

Secondly, think about what you really want. Your ex might never change. She wants you now, next week she might dump your ass again. Don't ruin relationships with people that really care about you for people that offer you drama, excitement, and strong (but toxic) feelings.

Do what you feel is the right thing to do, do what you think it's best, but take into consideration that, at 18, you might get it wrong, and it's normal and it's OK.

Good luck!