Trouble Ejaculating, Painful, Unable To Insert In Vagina, Used Condoms. Where Does The Problem Lie?

My Girl friend and I started having sex recently well we have tried. for our first time. We are both virgins. My penis has only gotten inside her vagina a little bit. Her Vagina has been stretched and opened up just a bit. But for some strange reason we can't get it in completely. She feels pain as its normal since its the first time. But we try and it just doesn't go in. We both get very turned on before time of inserting comes but it either slips out or it just doesn't go in so we both get frustrated and stop trying. I was wondering if perhaps there's any tips or advice's about how we can get it to happen already? Is there any special or specific thing that needs to be done? And for some strange reason with the condoms it seems harder to get the penis in it slips too much. Please if u can answer my question and help find a solution I'd be truly grateful.