I'm no medical expert by a long shot but you'd need to taper of the diazepam very very slowly. There's quite a lot of resources out there on the web but I'd really try and find a doctor who is able to help. I'm so surprised at everyone's response! Diazepam is a class C drug at the end of the day.

Benzodiazepine withdrawal isn't very nice and I don't think the sertraline will help much.

Benzo Buddies
The Ashton Method