Do your push ups on your knees. The only push ups that can't be done on your knees are the "pike push ups" and those don't come in until Phase III I believe. Go to beachbody.com and search for "bands over doors" or something along those lines. There is a way to tie a loop in a sock, hang it over a door and run the band through the sock to do the exercise. But if you are going to negatives with a doorway pullup bar then that is moot.
Rule #1 for P90X - DO NOT GET DISCOURAGED!
Rule #2 for P90X - MODIFY, MODIFY, MODIFY. If you currently struggle with doing the exercise modify it so you can. (Pushups on knees) If you do this and apply yourself, you WILL be cranking out pushups like nobody's business.
Go to beachbody.com and use the message boards over there also. There are thousands of people on there who are in various states of X'ing and all of them are more than willing to help you on your journey. Usually someone will start a "Starting on X date, who wants to join?" If that date is near yours or yours, join the group then BAM! there is your support team.