I don’t know if I can do this anymore

I (24F) met my partner (32 M) 4 years ago. I always knew he had a son (9yo) and I knew he had him pretty much full time and that his ex was high conflict. I knew all this but I guess I never really knew exactly what it was going to be like long term and how much strain SS and HCBM would put on my relationship with my partner.

My partner and I have 2 kids together (2yo and 3mo)
I know I’m young and probably should never have started dating someone with a kid but I can’t change that now and I love my partner dearly he’s the best dad to all 3 kids and I love the little family that we have created. The part that I can’t deal with anymore is constantly being the one that’s blamed for literally everything and always being seen as the bad guy by my SS. There is absolutely no doubt that HCBM talks absolute shit about me and my kids to SS when he sees her and I try my hardest not to let it get to me but I am really starting to struggle. My partner and I have tried our hardest to give SS the best upbringing we possibly can. Making sure he is raised to be a respectful young boy and knows that he is truly loved by everyone in the family. We go on frequent family outings and spend lots of time doing things that we know SS will particularly enjoy and my partner and SS spend frequent one on one time together as well, but for some reason everything we do just isn’t enough for him. He never says thank you or please unless we remind him 100 times, he’s constantly complaining about how he never has fun here and says that we ask too much of him. The only thing we ask of him is that he is respectful towards all members of the household and that he makes sure he picks up after himself. We’re not asking him to do slave work we just ask that if he makes a mess to clean it up and if he needs help to do so then ask and we will. Really quite simple but to him it’s child abuse, at least that’s what his mother has relayed back to us from what he is telling her. Unfortunately HCBM’s parenting style is to not actually parent at all and to let her 3 children all under the age of 12 run free with no consequences or responsibility. I mean this chick has her 4yo 9yo and 11yo staying up till well after 2am on school nights watching god knows what on YouTube and other streaming services, feeds them junk everyday like I’m talking to the point where my partner and myself once asked if SS wanted to get mcDonald’s for dinner as a treat and he said no because he had it 3 times in that same day with his mum!!!! And then when we asked what they had been eating on the other days he was there and he responded with mostly just 2 minute noodles and Maccas!! He was there for 3 nights!! So much junk!! Anyway she also believes that telling children about adult things is appropriate. For example she has explained what different sex terms are to SS with no context just a random conversation that she decided to have with him!! Like WTF??? Anyway all this shit has now lead to SS believing that his mothers house is the fun house and that our house is way to strict and we’re all so mean to him. SS is also now suffering with sever mental health problems which my partner and I are taking the steps required to help him with this but my god is it putting strain on our everyday home life. I now can’t even say hello to SS without some rude snarky comment coming out of his mouth and don’t even get me started on if I ask hike to pick something up or put something away, that turns into a full blown screaming tantrum with door slamming and yelling profanities my way. I just can’t take it anymore. HCBM is also threatening to take my partner back to court because SS keeps telling her that he’s so miserable and unloved in this house and that we all hate him. Court literally cost my partner every cent he had last time they went so this time it would mean my partner would have to give up his own business and start working 2 jobs just to stay a float. I’m also becoming super anxious about the kind of influence SS is going to have on my 2 babies and I now feel like I need to protect them at all costs. I don’t want to end my relationship as I love my partner so much and we really do work well together as a team but SS and HCBM are really taking a toll on my mental health, my partners mental health and just the overall dynamic of our relationship.

Idk if I’m looking for advice, to hear other peoples stories or if I just needed to vent but yeah just putting this out here in hopes I can find some kind of light at the end of what seems to be a never ending dark tunnel.