Me and my stepdad were often at loggerheads and there were a lot of rows. I think I avoid conflict for this reason but rather than taking myself fully away from it I sweep it away until next time
This is it, @sleighbellsjiggling. All your power to change your situation and your future outlook is in this recognition. You've been conditioned as a child to feel things, negative things, and then minimise them. 'My feelings aren't the important thing. here', 'I'd better not express how I feel, because it'll cause more trouble than if I just surpress everything'
This was your education in your formative years: 'My feelings are not important'. That's what's leading to all your 'I'm just being dramatic', 'I feel so pathetic' feelings now. After all, anybody who makes a fuss about something so unimportant as their feelings is just being a bit silly, right?
When we turn into adults, we parent ourselves, instead of needing other adults to do it for us, and we follow the example we've been set. You don't have to keep following it, OP. You can stop. You can change it.
When I realised how to do this, it was a realisation that my feelings do actually exist, they do actually need to be taken care of, and it was my responsibility to perform that role. To start with, it was like looking after a child; my inner feelings would strop around, and stomp their feet and say things like 'I don't WANT to go to the party!!'. I used to minimise that, and just go, because I 'should', according to some set of rules that I realised, eventually, didn't actually exist. These days I hear the tantrum voice, listen to the message, and act accordingly. You have to take the 'tantrum' out of it to present it to the outside world, but the end result is the same: respecting your feelings. 'I can't make it to the party this evening, but thanks so much for inviting me. I hope you all have a lovely time! Are you free for coffee next week?'
No apologising. Just respecting your feelings, and presenting them calmly and politely. And the only lesson in boundaries is that if somebody tries to prevent you from doing this, or makes you feel bad for it, you distance yourself from them.
So much of it is about recognising who is pushing your boundaries, and staying away from them, in order to have a life of un-pushed boundaries, and healthy relationships. In order to respect your kids and yourself, you need to leave your partner.
I was the child of a relationship like yours, and I had counselling in my 40s after a string of short term, unhealthy relationships. It took so many years of feeling crap/low self esteem/crossed boundaries, and a year of expensive sessions, to train me, in my middle age, out of what my parents taught me. You are there now. Your children will be there in later life, unless you choose to break the cycle, and demonstrate to them that walking away from unhappiness is the path to happiness, rather than trying to stick around in hell, make tweaks, and be happy there.