I'm not sure if I'm depressed, but I feel horrible and have done for a while now.

I'm not sure if I've actually got some kind of depression, or if there is anything a doctor or medical professional could actually do to help me. Around about every month (ish) I just start to get really miserable, fed up and I overall just hate life. Its been happening for about 3 months now and its really beginning to suck. For the past 5 days I've struggled to sleep as much as i usually would and i end up lying in bed for hours just hating life. I don't know if I'm depressed or if there is anything that I can do, but i just really want this to stop. All help is much appreciated, thanks.