Firstly, you're really brave for talking about how you feel. Acknowledging it's not a nice way to think is a big thing and you clearly want to fix it otherwise you wouldn't be asking.
Never feel bad for how you feel. Our feelings are what they are and fighting them only causes internal resentment. You need to find a way to move forward positively for all of the members of your family. And by taking on a partner with a SC that means them as well.
Secondly, a few things to try;
Research Nachoing. Not your kid, not your problem. Some of it can be extreme and you may need to start with extreme to gradually work up to being in a better place but it's great when boundaries are all over the place and it's causing resentment.
Talk to your doctor. There could be a bigger picture relating to your own health. It never does any harm to get bloods and general health checked up on when we notice our own behaviour is out of kilter.
Talk to your OH. If this means writing a letter, or voice notes, do what you need to do. Don't leave because it's an easier option than working hard to push through. He probably doesn't realise how hard it is for you and it is difficult for people to understand if we don't tell them.
Get a court order or parenting plan in place. If Mum is someone who will sit down and talk then get some schedules/routines firmed up. If she isn't, then go to court to get fixed days including holidays so you can plan. I'm an uber planner and SD's mum is the complete opposite but you can get there by staying consistent and holding your own.
Journal. You might find it helps to see if there is a pattern of behaviour by anyone in the house that is triggering your reactions. It could be a perfect storm of things that just tip you over the edge.
Your time. Find time for yourself and your 3yo to go and do things on the weekend visits. Explain to OH that you would love him to join you but understand that sometimes that's hard with the 8yo. Ask to create some time where you do things as a 3 and explain to him you want his bond to be as strong with your 3yo as it is with his 8yo.
Find common ground. This can be hard until things are on a better level but if you can find something that you both like doing or watching/playing try and make it just yours and the 8yos things you do. OH can use this time to bond with 3yo. Lego is a great way to interact with this age, even if you build something separately you are still spending time together and the 8yo will appreciate that, it may not seem it, but they do.
It's not going to be easy but I'm pretty sure giving up is far harder in the long run and not what you want for your 3yo. Things can get better but it takes the effort of everyone - even the 8yo, you just have to guide them.