Read the information at this link throught Plants & Drugs/Cannabis/Effects:
Erowid

It says cannabis cessation can often cause a problem called anhedonia, or lack of pleasure/enjoyment in life, which is basically another way of saying depression. And if you read the other information on the site, you'll see the depressive reaction can indeed happen. I'm fairly well convinced it happens in people who're prone to depression anyway, but when you have that reaction, I know it's miserable no matter what's causing it.

If you're actually feeling so bad you're making suicide attempts, that means it's time to see a professional ASAP. You can also help yourself by forcing yourself to get some exercise. That'll help counter the depression you're experiencing. It's the hardest thing to do when you're low, but it makes a tremendous difference. Seriously, though, if you're so low you're thinking about attempting suicide or already have tried it, it's time for some psychological/medical intervention. Good luck to you!