I cannot put my penis into her. She's a virgin and every time we try having sex, I loose my erection

This is possibly because, with some girls, fisrt penetration of the hymen can be difficult or painful for them. For the male, squeezing or pressure on the head of the penis can reduce the blood flow, which klls the erection.
Two options exist. Either take Viagra or Cialis an hour or two before you attempt to penetrate her. Your penis ahould then be rock hard. The other option is to get yourself a dildo and use that for her first 'penis'