Relationship help - over thinker

My bf and I have been seeing each other for 5-6 years now. Since we started dating I’ve noticed from when we first started dating my libido has kind of died off, he feels it’s because he’s not good looking and I’ve made it clear it’s nothing to do with him and and it’s more of a personal thing. I suffer from a bit of social isolation where I don’t really have friends/anyone to talk to outside of my partner. He’s the best partner you could ask for and is very caring. He has brought up marriage a few times in joking manner and it always puts me on edge. I’m younger than he is and I feel it shouldn’t be making me feel this way for how long we when been seeing each other. I’ve put him through a lot of my own personal stuff and I feel if after 5 years I can’t feel comfortable with marriage then I shouldn’t be dragging him on, I love him but I’m lost.