The only reason stopping pickup would make you lose confidence is because you were only changing your behavior. You only had situational confidence based on external validation. Your confidence was directly dependent on your immediate results.
Underneath it all, you still had a lot of insecurities and codependency that you had not entirely worked out, so your true colors came out once you removed the validation of pickup and put yourself in a relationship.
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I met this girl who i felt completely in love with, and as always when people are in love, they get insecure.
This is a false assumption, only people that struggle with insecurities and scarcity mindedness get insecure. As far as my experience goes -- when I fell completely in love with my girlfriend I didn't feel insecure at all. In fact I felt more secure. Same goes for my girlfriend. Neither one of us struggle with codependency.
It's a common experience in pickup when a man learns techniques and lines that give him the illusion of confidence. But it's fleeting and weak. It's like the man that built is house on sand --it easily crumbles under stress.
What you needed to have done, or possibly still need to, is to work on your emotional boundary control, which will prevent external sources from gaining too much influence over you. You need a stable source of self-validation and true confidence. This means you won't <i>need</i> validation from women to feel happy. You will love yourself just the same regardless of whether or not you have a girlfriend or not.
Shedding yourself of emotional codependency is the real game winner. Becoming internally validated makes anything pickup related almost entirely irrelevant. You can graduate from the community and move on.
I have been in a relationship for about 2 years now and I did not lose my "game". I'm constantly growing, gaining confidence, gaining a better understanding of myself and improving my life. So if anything my game is better now than before. Now I might have lost some social momentum, but that's very easy to get back.