For what it's worth, being happy in a relationship and being happy alone are both muscles that need to be worked out. It's good to learn how to create a life for and by yourself without the need to run to a suboptimal relationship, but it's also good to learn how to be happy in relationships even when the other person can't read your mind. Different people lean different ways and need to hear different advice, this is just my attempt to make it more universal.
Humans are social creatures, and isolating yourself will lead to a need to get back out there eventually, but then you'll be annoyed by the "offer" because nothing compares to the highly optimized entertainment you can give yourself, and then you'll go back home and put a few more bricks in the wall. It can be a vicious cycle. It's an even more vicious cycle when you don't like being alone with yourself, so you get back with the crappy ex, and then you hate yourself and being with yourself a little more.
I've been on both sides of this. I'm an odd duck. Most people aren't too into things I'm into, entertainment-wise, and my husband is one of them. I'll never be able to share some stuff I like with him because he will never get into it. I'm sure he feels the same way about some stuff he is into. We do share a lot of things, we're not worlds apart, but it's not like he's "my other half" - some friends come closer to that moniker than he does, and I can't compete with some of his friends either in that sense. I prefer hiking on rainy days, and he prefers hiking on sunny days. I like to be as close to the front row as possible when we're seeing a concert, he wants to be out of the crowd as much as possible, so every concert is a compromise. I'm a flexitarian, he will not eat a single meal without beef or pork - I kid you not, his favorite bisquits are made with pork fat instead of butter
But when he's away or I'm away, those daily annoyances just melt away, and I only remember the fun we're having. And being able to have sex and cuddle with a compatible person whenever I like is a big bonus.
Maybe it's because I'm in an extreme situation - we moved away together into a country where we didn't know anyone and we still don't know too many people. It's expensive to fly back and forth so most of our friends will never visit us, we have to come to them. Because of that, I look back on all the hangouts I missed because I wasn't in the mood to talk or all the annoyances I had sharing my daily life with people I now miss, and I want to grab myself by the shoulders and give myself a gentle shake, just to nudge me out of my introvert shell. Most people will never be in this situation. But if you recognize yourself in this, maybe this is the advice for you.
I used to share a room with my brother and every time he went out it was like a holiday - I can finally be alone! Do all the things I want all by myself! Well, turns out what I like doing by myself is very average. I like to write things - not anything loads of people like to read, just music reviews and personal blogs and stuff. I like to listen to music. I like to read the news and watch cartoons. My brother is also very average - he follows sports, he watches movies and he plays video games. It's all just... consuming content. It's not like we were working on a creative hobby or our health or traveling. Just pleasing ourselves with content that goes straight into the brain to trigger happy hormones the easy way. But when we did that together, those were the most fun times I remember and that I would kill to have more of, instead of having us both with our own pair of headphones enjoying shit on our own desktop computers, separately. Ten years ago we stayed up all night listening to a Kansas compilation and discussing it, and I still vividly remember it as one of the best times I had with him. Even though he's an annoying know-it-all who pretends to know what prog is