Help me please

I need major help. I am very confused to say the least about my situation. 
I normally have long cycles around 35-40 days. I am currently on day 43 of this cycle. 
My last period was on 6/19 - I have not had a period in July. I am not pregnant. All tests are negative. My ob/gyn prescribed Provera and said I may begin to take  it "whenever I feel that I want to" but I have to take it for 10 days and then I should begin my cycle. 
Here's the question. I don't know how long I should wait before taking it or if I should wait to see if my period arrives naturally. I have missed a period before and started as usual the next time I was due...
Then to make things more complicated I don't know what I will ovulate again. If I take the Provera will I ovulate the normal 14ish days after that chemically induced period? Or will I ovulate my normal time? Or will I not ovulate? 
If feel like I am about to have some type of mental break down. I need some major advice. :-(