What am I supposed to do..?

I am 15 years old and I am currently having sex with a 18 year old. 
Yes yes I know he could go to jail, trust me we are both aware of all that's at risk. We are being extremely careful, I am on birth control shot right now and tomorrow I am getting Nexplantion. I'm just extremely confused with this situation, nothing makes sense to me and I can't talk to any of my family about this they have no idea I am not a virgin and I know if they knew about us they wouldn't let me see him anymore. We've been having sex for about two months now, but from the very beginning we said we didn't want a relationship and we were just having sex, no strings attached & we have do's n don't 
Well recently he had sex with his ex and I wasn't upset about him having sex with her I was upset that he had promised he'd tell me if he had sex with anyone else and I confronted him about it and he apologized n I forgave him like nothing and had sex with him again... 
I don't understand what I feel? Like I know me n him just can't happen, he's 18 going off to college in 4 months and I'm his sisters best friend (she knows about us), it's just not a possibility and on top of that he doesn't want a relationship anytime soon... Which here's another thing I don't understand, he still wants to have sex while he's in college... 
So I just stuck with everything I just want him. When we are together we just click and it's like I've known him my whole life
He makes me feel so insecure & afraid but so alive... I just don't know how I am supposed to feel about this? I'm not sure I like him but I don't want this to stop.. Help?