Open Question: why can i not use a condom?

i need some advice with this..
ive never been one of the guys that gos around banging a lot of chicks that’s not me. but i have found a girl that i like were dating and we started havin sex. THIS IS WERE THE PROBLEM STARTS.. during for-play every thing fine i have an erection were both turned on so we go to move on to full intercoursee and place on the condom(not wasting any time or loosing the mood) it doesn’t feel to tight but what do i know .. any way after a few short minuets even wile me and her are both still highly stimulated i will loose my erection. i have had intercourse with her with out the condom and everything was fine showing two things im atracted to her an im not gay eliminating thous to options why can i not keep a hard on with a condom on?
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