booyeah,
Welcome to The Gym. If you're looking for support, you'll find it here. About your questions...
Should You Go Longer?: That's really up to you. Do you want more? If yes, then go for it. If not, then your girlfriend is going to have to accept who you are and who you want to be.
About you trying PE in the past: What was your routine? Lay it out so the members here can throw some suggestions your way.
About getting to 7 inches: That's a 4.5 inch gain. You're right, that will take a long time. I'd put your goals much lower at first. Shoot for 1 inch at a time. Depending on your age, you might be able gain 4 to 5 inches over a period of many many years.... That's the best way to conquer PE anyways (thinking about the long term). If you really are unsatisfied with your penis size, and not just because your girlfriend is, then in the long run you will be happy that you put the time and effort into making your penis not only bigger but also harder.
Good luck!