Why dont I feel able to love?

Honestly this might be hard to explain so im sorry if you dont understand. But if you do and see my problem right away pls lmk!

Basically i’ve been in many relationships and I feel as if when the new exciting crushing part wears off I end up just tolerating them/liking them as a friend.
For example, im currently in a relationship and he is head over heels for me, constantly missing me and seemingly very attached/ clingy. I totally do not feel this way about him. I mean i would be completely fine if he left right now and probably rarely have a thought about him or what he is doing.I do like him though i think because i dont exactly hate him.

He is my first everything so i feel as if i should defiantly feel some type of way but i just dont.

This has been the case in many of my relationships, I just kind of lose feelings or interest and let them stay around until they are visibly done with my distance or if i can no longer tolerate them.

I dont want this relationship to end but I just cant find it in me to love him.