You sound like you are a pretty self-aware person, especially since you can easily acknowledge when you are thinking all-or-nothing or black-and-white.
Given how conflicted you feel, the more you try and push yourself into one box or another it will likely keep stressing you out. I don't think it has to be like this, you have expressed that sometimes you likes girls and sometimes guys, and this attraction can vary across different situations. In my humble opinion, it sounds as though you are letting things such as your porn habits define who you are instead of just existing and following your gut. Just because you are with a girl does not confine you to living a "straight life", likewise does breaking up with her commit you to a "gay life". Any bisexual person in a monogamous relationship ends up with one sex and not the other at the end of the day. I hate to be cliche, but this is probably why society is better understanding gender/sexuality as a spectrum - it really is not so clear cut for anyone!
Perhaps if these feelings are so strong, you should let your girlfriend know how you are feeling, because if you don't she can never be there for you when you are struggling. No matter the outcome, you and your girlfriend owe it to each other to be honest, the same way that I am sure you would want her to tell you everything (please correct me if I am assuming wrong!).
I wish I could be of more help, but with these situations every person's story is so different and there is never a right and wrong answer.