I haven't tried 5-HTP...but I just read "The Mood Cure" and the author recommends it for people who have low serotonin levels. She has some very specific tips. Sounds like maybe you didn't get this info from your doc....
Take it in between meals so that it isn't competing with other amino acids to get into your brain.
Take it 2 or 3 times per day--it only gives a short term lift.
You need to have sufficient Calcium, magnesium, vit D, and B vits because they all help to make the neurotransmitters that assist in converting tryptophan to 5-HTP and then to serotonin.
It often doesn't work for people who also have thyroid issues. Aside from that, some people (she thinks 15%) seem to do better with tryptophan (which converts to 5-HTP) instead.
She seems to think that it should help you feel better within 30 minutes of taking it and if it doesn't, you should increase your dose or possibly that is not the right solution for you.
Hope that helps and hope you feel better!