What can I do to regain confidence, self-respect and learn to internally validate myself and stop being a bitch?

Context:

M24. Broke up with this girl 5 months ago with whom I had an on/off relationship for half a year before that, where she was a plate/fwb of mine. As time went by caught feelings. Found out she hadn’t been sexually exclusive to me in the early stages. She possibly was BPD and she lied a lot. Even though I had experience with women, I was fooled.

Walked away, ghosted. She did everything in her power for me to take her back (without me asking gave up flat keys, phone location, removed guys on socials), just to prove her loyalty.

I did not know better at the time and thought logically I can see her POV. We got into a LTR. Anyway, once I took her back I could not shake 24/7 rumination on where I fucked up (instead of seeing her for what she is). Gradually (since I instinctively cucked myself), I subconsciously began hating myself. I started losing frame. That shit mentally destroyed me. In the end, before I completely lost myself, I broke up with her. She still wants me back.

I have her blocked everywhere, but the fact that I took her back AFTER finding out she hadn’t been mine and mine only (blue pill I know, but instincts nonetheless), made me feel so emasculated that I cannot shake the feeling of hatred for myself ever since – it has been half a year now.

I fucked only 1 other girl a few times after that but that shit didn’t help me at all so I didn’t want to bother anymore with women – because that girl destroyed my confidence and made me feel insecure (I know I derived validation externally with her – and that’s what I seek to change here and am asking for advice).

It got so bad to a point where I decided to quit all vices, I quit smoking after 10 years, haven’t had a drink since. I have a business and it is going great. I train MMA every day and now took my work remote and even went to Thailand to train Muay Thai 2x daily.

On paper my life seems very great and on an amazing trajectory. But in reality, I am a depersonalized robot who hates himself and lives in constant emotional pain. I sound like a bitch and I know that, that’s why I put extreme pressure on myself to beat this bitchiness out of me.

Yet nothing helps?

I don’t understand why I miss her validation so much and the perfect dream RP-relationship scenario I had built in my head which was a fantasy, while way before I didn’t even give 2 fucks about that girl.

I don’t even know if it is oneitis or if it’s just some internal problem I have. It’s been so long it’s pathetic. I do not wallow in that pain, I try to stay active 24/7, but I am just tired of this constant pain I have in my chest from the breakup.

Do I really have to GFTOW? Yet, my confidence is completely killed to go after new girls even though on paper I am much improved since. I would like to fix myself first AND then go game new girls.

What I also do not want to do is seek a shortcut – to fix this issue with more external validation without fixing the core problem. Or is it truly the only way and internal validation doesn’t really exist (and TRP guys who say otherwise just cope)?

Or should I just stick to this disciplined routine I have and one day these feelings will magically fade?

I WANT to learn internal validation (if it even is possible) and I am open for any suggestions to achieve that.

TLDR: Broke up with a girl who fucked someone else back when she was my plate (for whom I later caught oneitis). Allowed to attach my ego/value to her actions. Cannot regain confidence ever since and do not understand why. Want to learn how to internally validate myself and regain confidence, without looking for quick fixes, before I jump into gaming other girls after a long monk mode. Is GFTOW really the only way?

I WISH I could feel anger/rage again like the first time I walked away. After I took her back she gaslit me and my emotional brain blames myself for what she did (her hypergamy), i.e. I was too cold, non-committing, jerk, distant with her and shit.

Any advice helps.

Thank you.