You need to go back to your doctor for some other options, the Provera obviously isnt working.
First, it is always best to know when you are ovulating if you are trying to conceive :
American Pregnancy Association : Getting Pregnant: Ovulation Calendar
Hopefully this will help a little.
Other than hormonal treatment, the only way you can make your periods regular again is reduce your stress level. Take some time throughout the day to just sit and relax. Watch some TV, listen to music, etc. Also, if your diet isnt the best in the world, fix that. Your diet can also highly interfere with your menstrual cycle. And finally, do not over exercise. Over exercising can throw your period off balance as well.