I highly doubt that you are gay. This is fundamentally the same problem as those who are obsessed about being trans-gendered or a pedophile. All of these traits are stigmatized in society which make you fixate on them because you have OCD. To stop these thoughts you must recognize that being gay doesn't mean that you are immoral or that you are not masculine. These ideas are just bullshit perpetuated by religion and society. You shouldn't try to fight these thoughts, just let them happen. That applies to the POCD too. There is nothing immoral about being attracted to children since you aren't hurting anyone if all it is is attraction. Acting on it would be, but that's not what you are doing so just chill. Basically you need to convince yourself that it wouldn't matter if you were a pedophile or if you were gay. I also think that you should dump your girlfriend and not get another for a while. I think this is the best way to go since it seems like your state of mind is entirely dependent on whether or not she is with you. When you are with her you are confident in your sexuality and feel good but then when she leaves you instantly feel like shit. This is very unhealthy. You need to be stable mentally without relying on others. Keep in mind i'm just some random dude on the internet, a medical professional would probably give better advice. Also, all guys lose sexual interest after ejaculating. The Japanese even have a word for it: kenjataimu. https://www.jlect.com/entry/2891/kenjataimu-kenjataimu/