Helping gf finish

My girlfriend has never orgasmed in her life, says she never masturbates either. I try to go down on her but she always pushes me away and prefers penetration. I sometimes try over the underwear clit rubbing which she seems to enjoy, but it always leads to penetration. She likes g spot/cervical stimulation but I can never last long enough to make her finish (though she says she has come close). I’m maybe lasting 10-15 minutes post-foreplay, with stopping and starting for control. Any advice? She’s inexperienced and perhaps self conscious of her body