Oh OP, as someone who has been in a similar situation many years ago I remember posting here for help. Everyone told me the honest truth and I was so defensive of him, even though I knew deep down they were right. I stayed with him and put up with his man child ways for many years, so many years wasted on him. Like you, I had parents who have stuck together through thick and thin so thought by me doing just that with my ex that's just what you do in relationships.
But respect and loyalty should be MUTUAL, and you KNOW with this guy it's not.
Now since my ex I've dated someone totally toxic who needed blocking. As someone who is a people pleaser (aka a doormat), I just allowed this new guy to totally trample on the self worth I was building up. I have now gone no contact with him, and it was SO hard but going NC, and being single, are the best things I ever did.
No Contact is actually step 1 of building self esteem for yourself. It may not feel like it right now but it's the FIRST time you've put yourself before him in so long that you can hardly recognise it's a good thing. The guilt of blocking him is something you really need to address. The excuses we all give - maybe I should message him to see how his dad is/it's his birthday/I need to get something back from him blah blah blah excuses.
Being single is NOT scary, it's actually pretty brilliant. Right now I have zero drama and can put all my focus onto my kids and myself. Pour all that care you give for him into yourself. Imagine that was your child in a relationship with someone taking total advantage, treat yourself like you would your child and pick yourself up and dust yourself off.
What I find helps is to write little post its and stick them in places I will read often. One for me says 'You made the right decision to block him' another says 'Well done on putting yourself FIRST.'
I realise now just how much this drama was taking over my life. I thought my kids were fine, I thought I was fine. But actually it was affecting my health, and I wasn't fully 'present' with my kids even though I didn't realise at the time, but my mind was always occupied with what he was up to or why he was treating me so badly.
It's time for change now. Just realise if you go back to him you will NEVER get any respect from him. He will be secretly laughing knowing the girl he's used for so long is ready to be used again.
www.baggagereclaim.co.uk
www.postmalesyndrome.com
Sites that help with NC.^