What do you call premature? You do know that 5 mins is perfectly acceptable? I've had many a chat with women who've had to teach their partners that banging for an hour isn't what they want or need.
Her not having had an orgasm is not on you. It's up to her to figure out how she likes it and teach you - not the other way around. Also, most girls will achieve orgasm via manual or oral stimulation, so a quick root isn't going to make much difference. Do not underestimate the power of good, long foreplay.
Anyway, just have a quick wank before you leave. Then you'll last longer when you're with her.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.