5-HTP is converted directly to serotonin, which can then be converted to melatonin, hence the similarity of effects. Tryptophan is the precursor to 5-HTP, and may be safer to take, especially long term. 5-HTP is NOT SAFE to take with SSRIs or St Johns Wort, but tryptophan is ok. If serotonin levels start to build up, the body can use it for other things, but 5-HTP converts straight to serotonin. There are some concerns that taking it long term could be bad for your heart, so if you try it and find it works you might want to use tryptophan instead.
Another amino acid you might want to try is tyrosine. You'll find this in the health food shops body building section if it's not in the mental well being section. It's the precursor to dopamine, one of the brains other "feel good" chemicals which is also involved with alertness, motivation and stamina and is the precursor to adrenaline and noradrenaline, hormones which regulate the bodys' response to stress. Depending on your brains' chemistry, you might find this as helpful or more helpful than 5-HTP, especially if you are or have been under a lot of stress. People who don't respond to 5-HTP, or who respond poorly to it, often respond well to tyrosine, and they can work together well when combined.
Taking these supplements also can give you an idea of which herbal or prescription antidepressants might work for you if you find you need something stronger. If 5-HTP doesn't work, then St Johns Wort or SSRIs which act purely on serotonin are unlikely to do you much good. If Tyrosine works then Rhodiola rosea or a prescription antidepressant that boosts dopamine such as sertaline (Zoloft) or bupropion (Wellbutrin) might be worth investigating.