Overthinking and the power of your thoughts

I don't know if I have OCD, but I know for sure that I'm an overthinker. I also question if there even is a difference between OCD and being prone to overanalysing and overthinking, to the point where your entire day revolves around one subject, but that's another topic.
What I hate about this is how my mind controls my feelings and how I experience things.

The point I'm trying to make is how my mood/thoughts affects how I interpret what I experience and how I view my girlfriend. One day I think she's perfect, beautiful and I know she's the right one for me, and the next day or hour I think she looks unattractive and I question everything about us. The only thing that's different is what mood I'm in and how "in my head" I am at the moment. And if she does one negative thing I suddenly change "mode" in an instant, where everything becomes annoying, I no longer find her attractive, her body odor suddenly becomes unappealing to me. Our relationship used to be a constant 8/10, with few bad and fantastic days, but now it's like I've become bi-polar about our relationship and it's either 2/10 or 10/10.

I used to be really pleased with my girlfriend and found her very attractive. When I first saw her I would give an arm and a leg to be able to sleep with her. I enjoyed every second I spend with her. Sex, kissing and fooling around was exciting and fun and made me feel relaxed right after and energized the following days. I never thought about what I gave up by being with her or what her flaws was. I didn't even pay other girls much attention. Sure, I would recognize other beautiful girls, but I didn't about them.
It was really simple: I just really liked her. I didn't think about how her body compared to others' or about her boobs being small. It was her body, and that made it "perfect". I remember how I used to touch her boobs by "mistake" and how turned on that made me. Was the sex fantastic? No, probably not, because she was a virgin and not very "sexual". But I didn't notice, and it was very enjoyable and I looked forward to having sex with her. I'm not a very sexual guy either, so I didn't care about her not being more crazy and slutty. After we had sex I felt really relaxed and fulfilled. It made my day and also made us grow closer together emotionally. I knew I was going to marry this girl.

She started studying and we had to live apart for 2 years. This left me with a lot of spare time that I used to spend with her. This made me discover how bad my relationship to my friend really had become (even years before I met my gf). I was extremely lonely and isolated. I began PMO'ing more often and started having a lot of negative thoughts about my friends and how they treated me. The more I thought about the angrier I got. It occupied my mind and I felt my mental health deteriorating. I began questioning a lot of things in my life and became bitter and depressed. My girlfriend was the only good thing in my life and was so happy about "having" her as my girlfriend.

Eventually, my overthinking started questioning other topics, like my sexual orientation, after a period of feeling extremely inferior and anxious around other guys and men (at all ages). I was still crazy about my girlfriend, but suddenly I got this weird feeling around guys. The thought about being gay (without knowing) began occupying my brain for hours a day. Over time I realised that it's very unlikely that I'm gay, considering I never had any sexual attraction toward guys and when I had sudden periods of doubt I felt the same for every guy I saw, no matter how he looked.

However, I haven't been able to shake this feeling of doubting everything around me, which now targets my girlfriend. This has changed how every interaction with my girlfriend is:

I now often get drained after having sex with her (feels just like a PMO relaps), which I never did before. I question my attraction to her, obsess over her flaws, where I before didn't notice them and couldn't believe I had such a beautiful girlfriend. I ogle every women I see and get mentally aroused by almost all of them. I get overwhelmed by intimacy with my girlfriend, like kissing her and making eye contact. I often avoid kissing her or having sex. I resent her for not being more slutty, more sexual, not wearing make-up. This is the exact same girl which I, maybe just an hour earlier, found absolutely perfect.