I may be a guy but I do have a little info that could help...
Yes, the depo could have had an effect but not a permanent one.
Pregnancy had nothing to do with it in the long run but it was a contributor
my girlfriend has ovarian cysts, this had nothing to do with your loss...Physically. Mentally it could have depending on how you felt about having the cysts.
The cancer wouldn't have had a major physical effect. Just another mental one.
Anxiety and depression weren't directly related to this problem.
But here's the main thing-zoloft is actually an "upper" in the world of drugs (believe me I use to do speed I know my drugs) all anti-depressants are uppers, and so are anti-histimines. They reduce sex drive (even though for men they can cause massive long lasting erections) and often times affect sexual relationships.
I can't really give you a solution, but I can give you helpful information that could help with finding that solution. :)