Hey Mantis -- welcome to the boards! What you're describing is one of the most common things users write in to us to talk about on the boards. Our bodies are all so different, so sometimes it can be hard to tell someone exactly what will help in cases like this. One trend I do notice though is that it's very uncommon that there's actually something physically wrong or a disorder of any kind.
The first thing I want to suggest is perhaps the way you're masturbating isn't working for your body and it's time to explore more options. I think
Going Solo: The Basics of Masturbation will be very helpful because it give some tips on figuring out what feels right for you.
You're right, being on medications like that can make things difficult. I feel comfortable sharing with you that I'm also on zoloft/sertraline and experience something similar from time to time. Our brains are our biggest sexual organ, and this can lead to some difficulty when one experiences anxiety and/or depression. And sometimes the ongoing frustration of not feeling sexual pleasure can be the source of continued difficulties. When you're masturbating, do you find yourself thinking about how this has been hard and you're scared something is wrong?