I think you hit the nail on the head when you talked about: you resent dad for not spending enough time with you, for potentially cheating on your mom, and your SM being angry that your mom ‘took you away.’ In reality, none of these have to do with your stepmom they are all resentments that you hold on your dad/mom but take out on your stepmom. That is called scapegoating and it a normal response to difficult feelings. If you went to therapy, what they would dig into is forgiving your dad (and mom) and THAT is what would ultimately improve your relationship with your stepmom.

No kid wants to acknowledge their parents faults, but none of us are actually adults ourselves until we can see our parents as people and accept their imperfections. Maybe your dad did cheat on your mom. Maybe he wasn’t as involved as he should have been. Maybe your mom did take you away from dad. With time, you’ll be able to reconcile those hard truths in yourself and see your stepmom as just another adult who was living her life with faults of her own, but separate from the faults of your dad and mom.

What you are going through is a normal part of growing up. We all have a stage where we have to forgive our parents (step and bio). You’ll come out the other end eventually.