Ah, i can see what youre questioning here. Also, strange your GP made that comment.

I have been to more than a few psychiatrists, and it is always trial and error. There is little science behind these treatments.

You can google the algorithms used by psychiatry. They vary slightly by hospital, organization, but its pretty much the same.

There is a genetic test you can get that lists propensities, but they only offer vague guidance and dont take co occurring health problems, other medications, and other things into account.

Trial and error can go on for years, sorry to say.

Prozac can be activating for some, you could maybe switch to zoloft or celexa. PTSD is more difficult to successfully treat than anxiety.