I don't get it, anything really

Well, I am the type of person that is extremely outgoing and friendly to everyone. Except when it comes to relationships and who likes who. I can talk to girls with great confidence... at least as long as we aren't dating... I am perfectly fine til I get into a relationship... and then it crashes... because of me and I don't know what I can do to fix this. My mind keeps switching from, "Oh, just being with one person sounds nice." You know? settle down and stuff? But the other part of my mind when im in a relationship thinks," Maybe I shouldn't have done this... maybe It would be better to date around." and also when I am in a relationship... I can't talk to the girl worth crap... I just run out of stuff to say... and my ex told me I show off too much... I try too hard to impress. I'm only 14 but I don't know what to do.