Hey Joker,
It sounds like you're having a rough time right now. This forum is a great place to be in order to regain your confidence and get rolling again. It sounds like you're a young guy, and I can understand how it can feel so painful sometimes. As another young guy, I've been in your boat before. I've also written a previous post that you may find relevant:
Want advice for dealing with my longest case of oneitis
This desire for self improvement seems to have been borne out of an insecurity, some deep seated belief that you are not good enough. The fact that you went ahead and saw your ex and attempted to convince her that you've improved tells me that you weren't trying to convince her, you were trying to convince yourself. Before you even attempt to date another woman, this is the hurdle you need to overcome first. Forget your ex, she isn't the problem here. In fact, she shouldn't even be a party to this. The only thing that matters are your feelings toward her still.
I was faced with this when I lingered in the aftermath of my failed relationship in 2016. Much like you, Joker I went on a relentlessness march of self improvement. I was hitting it from all angles, and I did end up banging my ex again several times in the next 4 months. In hindsight I was trying to prove my value to her the entire time.
The hardest thing to do is realize that for most people, you will never be good enough. When you finally contend with this reality, it's going to sting a little. However, accepting this truth is the first step to your path of freedom. You're not good enough for them not because of you, but because of them. Their own sets of fears, and desires inform what makes someone good enough, which you have no power over. Your ex falls into this camp. My ex fell into it as well.
When someone feels that you are not good enough for them, you cannot change their reality. You can only let that person go, and let them go without resentment. I say this, because the root of unhappiness is to force yourself into situations where you do not belong. This causes a serious disconnect between your mind and heart, which is what you are going through right now.
To recap, you'll first have to accept that no matter how much you improve, you're not going to be good enough for her. That is a consequence of her own fears and insecurities, as well as desires that are not aligned with who you are.
Your second step to freedom is acceptance and forgiveness. Consistent happiness is based on an acceptance of yesterday, the hard work of today, and the eye towards progress tomorrow. It seems you've got parts 2 and 3 well enough in hand, so let's talk about the first part. Acceptance.
You're still having a hard time accepting that your ex chose someone else other than you. When someone chooses another person, they unconsciously choose someone that reflects who they are inside. Her decision to date a degenerate for three years tells me she has serious insecurities.
I invite you to view the situation like this. You dodged a bullet. I'll offer you this example. I just found out the other day that I got a girl pregnant. Not ideal, I know. However, this girl has solid values, a supportive family, and is extremely healthy emotionally. I dodged a bullet. Had this issue arisen with some of my former partners, I would be in deep shit.
Picture yourself accidentally getting your ex pregnant. The same girl who went out with her ex, and then fell in with a degenerate for 3 years. This is not the kind of woman you would want to be in a situation like this with. It's very clear she's not thinking of you, she's only thinking of herself. That could manifest all manner of problems.
Accept that you have a clean slate now. You've had three years of continuous self improvement and the chance to go out and find a woman (or better yet, women) that are more congruent with who you are now. The place you are is is one many men (perhaps even some on this forum) would envy.
Your ex is no longer part of your life. In order to allow yourself to accept this, you need to acknowledge all the unresolved emotions you hold toward her. You're very clearly still hurt, and I suspect there's anger there too.
Pain and anger are different sides of the same coin. They are permutations of fear. I'll come back to this in a bit, just keep the idea in the back of your head.
When I was recovering from my painful breakup, I came to a point where I realized I had a dumpster full of unresolved emotions that I had been pushing down for over a year. To let those lose, I advise you write them down. Write everything that you feel towards her, and then read it out loud.
I'm going to bet that many of items on your list relate back to fear. Fear that you're not good enough. Fear that you'll never find anyone better.
The first step to overcoming fear is identifying it. So I ask you OP, what are you really afraid of?
Figure that out, and then check out the Sedona Method. I'll allow you to be proactive and seek that out, because you have to participate in your own rescue. There's a free PDF that details it out there, I'm just going to let you find it!
After you've identified the fear, it's time to get back into the field. Start going on dates. Resist the urge to get serious with any one woman. By seeing the many different faces of women, your ex will begin to fade. You will experience highs and lows. Successes and failures as well. The whole process of self improvement in the women zone will help you heal the fears and limiting beliefs you have that relate to women.
Please feel free to reach out if you would like me to get more specific.
All the best my friend, and good luck.