Ok so been SD, I'm also a SM and a mum. I really looked up to my dad. In hindsight he let me do whatever I liked and was true Disney dad, and my SM had to take up the slack and was frazzled. This lead to me blaming her for everything, and her trying to get him to be a parent (shock horror he always let me know that he was stuck in a hard place - rather than actually taking ownership of parenting - ex when I had a house party while they were away, trashed the house and he said my Sm had told him to make me clear it up and he would have just let her do it but didn't want to upset her) - that was a particularly cruel stunt he always did.
I also had my mum telling me or hinting they had a affair and was the cause of her and dad breaking up and that meant I put a lot of blame on my SM that I couldn't put on my dad because I loved him .I didn't realise at the time and not so secretly hated her blaming them for my family being torn apart.
Problem is hindsight is a beautiful thing. I can now see that actually some people just don't work together and my dad as lovely as he was,was central to all of it. When you find out your parents have real flaws when you truely can say you are adult. It's ok to love them despite their flaws, but really look closely. Who's doing all the heavy lifting in your house, who cooks, cleans, food shops, ferries people around. It may be enlightening. Remember all that stuff takes effort and can really wear someone down.
I felt a lot like you tbh until I had kids. Then I realised the expectations that come along because your the mum and female so therefore have to do everything and Disney dads are somewhat acceptable. This rule also applies to SM. It would frazzle anyone.
If they did cheat then they are both 100% culpable. Not just her, remember he made the legally binding document to your mum.
She's not a good person for taking part in it but men don't get stolen, they make active choices. Set them both alight but don't start with her first right.
Personally if me and OH we're fighting really badly I would send kids off (both mine and his) for the day somewhere nice because kids aren't responsible or need to watch people take strips out of each other. It's not a healthy choice and I don't think children should have to feel unsafe because the adults in their lives can't get it together. I would also make it v clear to the kids this wasn't there fault and we loved them.
Sounds like you need to be more specific with what's she's doing that's upsetting you . Saying "I don't like you" is what a child says because they don't have the words to put it any other way. You do have the words your a smart girl clearly, use them. Work on the relationship, if you want to.
If you want to know the end to my SMs and mines story. She's amazing and I love her dearly. She's been always consistently there and as I turn old I realised just how much she did and does for me. But the teenage years and early 20s boy were those not fun.
^ I could be wrong all about the above and she's a wackadoodle. But since you did briefly mention she makes separate dinner for you because your veggie, I would guess actually she trying and I suspect it's a bit of Disney dad going on esp if he did cheat on your mum.
I just think your still a bit hurt and figuring things out. 18s a hard age actually a lot of change. I think you maybe still upset re the divorce (and that's totally ok) but you need to work through that.
I promise you when your a mum, you will get it a bit better and if your a step mum (which I never planned on being) you will understand just what a tightrope it is. I have messed up so many times being both, but i love those children fiercely even when they don't like me, because that's when they need to be loved even more xxxx good luck op