I am sure most of you PhD students know it, but phdcomics.com has great comics giving you the feeling..oh not only me this happens to everybody. I think this is true for most to-be-docs and is the matter of magnitude and time. and yes, if your project is going wel, you will feel a lot better and if project goes bad, exactly same feeling.

But best thing we can do is I guess, enjoy what you are doing and make some outside hobby, like sports, music. We did not do our PhD to make money, a decent money will follow anyway later. Think of long term..may be for betterment of human life overall.