Can my ruin my relationship?

My girl friend and I are both 17. I took her virginity and she took mine. I usually last about ten or so minutes during every ual . Sometimes I'll get soft and will have to go through two condoms to do it once. This have happened at least 8 times. I've failed to do it doggy a couple of times and move her around for nothing. We've only done it missionary but she's said that she would like to experiment. I'm kind of afraid to because I'm not the most equipped man if you know what I mean. Mine is about 2 1/2 soft and 5 erect. I'm afraid that I might keep disappointing her and she will leave me. She tells me she loves me but how much can a woman take. The last couple of times we would do it. But when I go for the second round I will get soft and we wouldn't have , and I would take her home. I would love to ask her if I disappoint her but I'm afraid to instill that into her mind.