my penis wont stay standing during sex

I've tried to have sex with three girls and all three times I've gone from hard to soft.. the last time I literally cried right there in the bed i was so horny before I put the condom on, she just walked out.
How do you guys deal with this? I cant remain hard when it comes to the crunch no matter how horny I am, I'm 22/M.

☻k