Fucking problem

I've started fucking my girlfriend (yes, and I'm here in 4chon). She was a virgin, and is still sore and feels some pain, so we use lots of lube. The problem is, despite having a considerably wide penis, this girl is kind of roomy, even though I know she was a virgin (pain, blood, etc.). Without much tightness, with a condom, and with lots of lube to absorb every possible trace of feeling, plus the added fear of hurting her and anxiety, I feel like I'm fucking air, and thus even though I can keep it hard for over an hour when I masturbate or have her masturbate me, once I start fucking her, my dick goes "call me whenever you need me". Didn't happen with previous encounters.

Halp.