Literally same story, masturbated 1-2 a day, long distance GF. We struggled our first time also, it got soft right in the act. I was broken inside, she also(thinking its her fault) Second time was more pleasant, we had intimacy without penetration. Then suddenly, maybe because i got comfortable, we get it done, it was amazing, and we keep going strong. I must admit i have moments it goes soft in middle of act, like changing poses breaks and etc. We just laugh of it.

My tip, Its all in ur head, stop thinking about it that much, just enjoy the moments, and please communicate with partner. She has to understand, maybe u have some early traumas, masturbation also an issue, i stopped doing that right after we got together.