I feel like I will never accept the BF as my daughter's step dad, role model, or anything else. Should I just let go and let ex's new BF into my daughter's life? Am I unreasonbly holding onto anger, or should I just accept the inevitable and do my best at being a great dad?
I understand your anger regarding this. Remember anger is a secondary emotion, and often the primary emotion is pain. It is hurtful to envision this other person a part of your child's life. The BF may not be "the one", and others may follow. As said, you have no control over what your ex chooses to do.
As time passes, it gets better. Acceptance is healthy. You can't force it upon yourself, but it will come if you encourage yourself. Remember, you are Dad. The only one. Your daughter will never forget that, nor replace you. Be her father.