I would also say be prepared for it to get worse before it gets better. My husband's ex wasn't great when we were dating, but she became fully psychotic after we got married...
Also, remember that there are others who have been in your shoes and you can come here to vent or chat or get advice anytime. Sometimes just reading about everyone else's ordeals helps me put it all in perspective (as a step-parent or otherwise).
-------------------- "Success is not final, failure is not fatal: it is the courage to continue that counts."