grow some balls.
Honestly, how do you think your girlfriend would feel if she knew the only reason you were with her is because you're too gutless to end things? Sure, she'll be sad for a bit. Then she'll get over you and move on.
As for the lab girl, I agree that there's probably rules against having relationships with students. I'm only a teacher's aide - but I know I could get fired for seeing a student.
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.