I am really getting fed up with my autistic stepson.
Is your stepson's autism something that has been diagnosed?
Yes I've talked to one doctor and he told me he has autism
Stepparents should not feel guilty for not instantly loving their stepkids, and any guilt felt could morph into a deep-seated resentment. Instead, you should accept the child without trying to force it and do what you can to communicate with your partner about the situation.
I don't think you understand what I'm going through.
Well I am also a step dad, and I know how it is feeling like you have to love the kids just like your partner does her own kids. Love is something that has to be built up, so just continue doing the best you can for the kids and let them realize you will always be there for them. Also, talk to your partner about how you are feeling.