I can't make her climax?

I have been with my girlfriend for one year now and I can't make her climax from full sex, she said she did once before on her life with her ex but insists it was a fluke, he is black and I got it in my head that maybe it was cause his tool was bigger, she said its not that and she can't even remember as they are broke up many years now. I can make her climax from foreplay but that's it, any thoughts? I've never had this problem before and I hope it doesn't ruin things cause all I can think about is this. Maybe I should walk away, but I dont want to as we love each other very much and there has been talk of marraige.