Friends with benefits with ex, or more???

Okay, I seriously need help. Im a 17 year old guy.
A while back I broke up with my girlfriend (who I'll refer to as "X" ) and I was really cut up about it. For 6 months i was a wreck then one day something in me just switched off and locked that part of me away. I started seeing other girls again but it was always missing that "something". Me and X had decided to remain friends and still talked occassionally, then about a month ago it changed.

We started talking a lot more often and one day we decided to meet up. Somehow we ended up making out. The next day we decided to meet up to figure out exactly what happened and ended up agreeing we shouldnt go out, deciding to be friends with benefits, and that if either of us started developing feelings for each other we would say it. All went good for a few weeks, then it started getting deeper. We started talking to each other every night about everything the way we used to, and lately whenever X and I meet up its not really been like Friends with Benefits. Weve ended up cuddling on sofas watching films together, holding hands talking as well as the normal making out stuff...

What exactly is this "thing" we have, what can anyone glean from this and should I allow myself to open that part of me i locked away? Should I tell her I'm starting to care for her again or should I just end it?

Thanks for reading, means a lot to me.

Maybe you both grew up a bit? Time for The Talk. Good luck.