I'm fairly sure I'm doing everything right to arouse her...she even initiates sex a lot of the time, so I'm certain she enjoys the physical side to it at least.

But after 15 minutes or so she says she's tired and complains it's uncomfortable...?
Is that normal? I haven't experienced this with a girl before.
When I feel her, she feels moist and my fingers can slide in perfectly. Physically it doesn't feel dry, but she says she doesn't feel wet.

This is only an issue once weve had sex the first time and try again. I actually feel really bad because I know it puts her through a bit of pain.
I'd like to think that it has nothing to do with me not being sexually attractive to her, because a lot wouldn't make sense if that was the case (such as how she does appear to want sex as much as me)...but I won't rule that possibility out if that's the only thing you can think of. Fingers crossed that's not it though haha.

My penis is average size as far as I know, so I'm 95% sure it's not because I'm too large for her.

What are your thoughts?
Have you experienced this before?