BackInTheGame78 is correct, but without providing tools to help you change, you'll just continue the pattern.
I'll provide a couple of personal examples of things I experienced when I was much younger. I used to wonder if I was cursed or if women were crazier than I thought.
Example 01 - It seemed like when I had a girlfriend, then (even when I was out alone and without her) all of a sudden I was receiving all these available and favorable signs from other women. I used to wonder why in the fvck am I only now seeing their outward signs of showing interest and why didn't this happen when I was single.
Example 02 - When in a relationship, I'd overlook a few minor offenses. I'd try to be lenient. Then, there would come a breaking point. I'd had enough. I was done. And then, all of a sudden, they'd become every positive thing that they were supposed to be. This happen several times in my youth.
Like yourself, at first I thought it was a sign of craziness with women. It wasn't an easy pill to swallow, but I was actually the cause of these problems.
As to women that were hitting on me while I was in a relationship, this wasn't a matter of validation/competition and them seeing me with another woman (this occurred mostly when I was out and about while alone). Humbling as it is to admit, I was subconsciously showing more signs of confidence. You see... I was already in a relationship and so I wasn't showing any signs of awkwardness or neediness to these random women. Apparently, when I was single, I was.
As to women I was in a relationship with... I was too accommodating. This was a turn off to women. The moment that I didn't care, written them off and focused on myself... they suddenly turned back into the girl they were in the first place. During the relationship, I had lost focus on myself and made concessions for women that didn't deserve this. In other words, I had subconsciously given them a 'green light' to behave in a manner that wasn't acceptable.
When I was able to understood the things that I was doing wrong (rather than blaming women) the above problems vanished and things started going better.