Why does this keep happening? :(

So there's this guy that I really like, and we've been flirting for a while now and we're finally going on a date soon.

But right now, just by thinking of it I get terrified! I'm 17(almost 18) and he's 19, so there's not much of an age difference at all.

The thing is, I've never had a "real" boyfriend before because every time I'd go on a date, I'd get scared and after that I'd push them away from me. I really like this guy and I don't want to push him away from me, but it seems like I can't help it!

Please can someone give me some advice? :(