I quit as a CFII/MEI, and it was really hard at first. Same as you, it was my goal for a really long time. I transitioned into part time flying and working an IT job, then finally just stopped flying when I realized I didn't have what it took from a sacrifice perspective.

This isn't the case with every pilot, but there is A LOT of sacrifice involved. You will probably have to move (more than once), you will be gone a lot, you will loose a lot of autonomy over your career and how quickly you move up, you will have HORRIBLE starting pay for quite some time, I could go on and on.

I spent years mad at the industry that I was a CFI racking up hours and I couldn't find a good opportunity flying corporate or something like that. I finally gave up being bitter when I realized it was ME. I wasn't willing to make the sacrifices - and it sounds like maybe you're not either. It's a hard realization, but it is a weight off your chest. It's not that you're not good enough, or that you have bad flying skills ... it's just that your preferences don't align with the industry.

As far as the girlfriend goes, it's fine if that's what sent you down this path. But, make sure it's because YOU don't want to make the sacrifices. Don't pass on the career because you think it will upset her. As long as your reasoning is sound and it's for you, there is nothing wrong with moving on.