Girlfriend Problems?

Me and my gf have been together for 4 months now and whenever we are together, everything seems great between us!

But there are certain things that bother me. She never picks up her phone, is very slow to respond to messages (if even at all) and often forget that she had made other plans on days when we were supposed to be together.

This might all be because she is only 15 years old (I am 20 btw) and generally a little "irresponsible" and perhaps she just doesn't really know how to respond or, when I try to make plans with her, turn me down when she doesn't really feel like spending time with me.. But I don't really know - It's hard to read her.

I know that girls need their space and that I shouldn't bombard her with texts/calls, but it's kinda clear to me by now, that if I don't contact her, I'm unlikely to hear from her at all. Last time I felt this way (which was about a month ago) I decided not to contact her until she had contacted me. I then waited a week before I couldn't resist anymore and she then told me something "felt off" in our relationship (which we then talked about of course) and she almost broke up with me.

Last time I was with her, was at a carnival event the day before valentines day (nope, she didn't even have time valentines day). This is partly because she has been sick a lot lately, but damn, I really want to se her soon.

What should I do D:?

ps. I'm going to ignore all those who just think I shouldn't be dating a 15 y/old in the first place >.>