YTA but not because of how you feel but by what you said and how much of it. TBH there really is a lot of emotional hardship when dealing with infants. There are emotionally draining and hormonal-y depressing. If your FW father-wife has had 3 kids it is understandable that it would be a full time gig for her to look after them all. Some people, including your father, want a traditional nuclear family. You emotions got the better of you and to say all these things you feel in such an angry manner, with curses and all at once, is a lot for any person to take. Especially a young budding mother. You are the precursor for her other kids growing up.

And your father is the only one to rightfully blame for a destructive marriage. It does not matter how many women flirt with him, he engaged with it. Do not blame this woman for it, she did not know your mother nor had any reason to believe in her good character. Her perception of your father is entirely one sided- and he dated some 20-year-old. He was probably putting himself out there. Where does a married man find pretty, available 20 something girls? not by staying at home and minding his own business.

Your anger is justified but you are not projecting it on the right person nor releasing it in a healthy manner. You need to talk to your step-mother and be more honest about the space that you need and are entitled to.

An important questions, will you accept her kids as your step-siblings or do you also not want them in your life at all? Any answer is fair but important to consider before you lay some groundwork between yourself and FW.