Something similar happened to me, only it wasn't against a mattress, instead i would lie on my back and apply pressure against my thigh. I realized around 16 that I was conditioning my penis to respond to that rather than the conventional.

When I lost my virginity I didn't get off at all, I actually faked it behind the darkness and the condom. The third time I had sex, just days later, I finally was able to climax, since then, I have tried not to jerk-off any other way but "normal" though it is still easier and quicker using pressure.

Good to see it's getting more attention these days, this was 6 years ago and I freaked over whether I could ever get off again.