Why can't I penetrate my girlfriend?

Hi, first I would like to specify that english is not my first langage..

Recently me and my girlfriend tryed to have sex for a couple of times but we are having a issue.Let me explain: We usually make out for a while kissing like animals and I usually end up fingering her( only 1 finger fits ). Dont worry i made sure she is ready for it she was the one to tell me she wanted to and no she is not a virgin . When I go for the penetration with my penis it seems like it is too tight or something I made sure she was really wet before trying i fingered her for a while she moaned like crazy she even told me she was wet enough for it, I usually end up unable to enter then I lose my erection .. I don't know what is going on (I'm a virgin by the way) we've tryed with me on top, with her on top but it rly seems like i cant get in there and I am very sad.. She told me that before the boyfriend she's had had smaller penis and that they put a bit of spit on the lips before going in (which I havent tryed just yet). I would appreciate it greatly if somebody came up the solution or a couple of alternatives to try.

Also i figured I'd add this.. Not too long ago I asked her to give me a blowjob which she did not seem comfortable with so i kinda wondered why? I did not push her or anything i was just curious and she ended up crying .. I told her i was sorry and I really didnt mean to make her cry and then she told me what she was abused by a uncle of her when she was seven years old .. I don't know exacly what happened but I think it was oral only.. I don't know if this might be related to her being so tight but i figured I would add this just in case.. I don't think i've forgotten anything so I will be waiting for your answers. If you need additional details or anything just post and I will answer when I check the forum

Thanks a lot to all of you who will take some time to answer this thread :'(