You’ve been living with this guy since you were only 15. Both of you have been deprived of important growing time. The teen years and the early twenties are a time to date different people and to find out who you are in a relationship, who is the kind of person who brings out the best in you, and who you can promise the same. That person is rarely someone you meet at 15 — unless you grew exactly in the same way (unlikely) and rate or you didn’t grow at all (also unlikely).
I suggest you trust your feelings and take a big step back from the guy you’ve been with. He’s been feeling restive and drawn to other people too. But neither of you should jump from this relationship right into another. Give yourselves the freedom to discover who you are as an individual first. Then take the time to meet a number of people to see who is a good fit.
Forever is a long time. You want to be sure of your choice.
I wish you well.