1) Bad press. Taste is subjective, a critic's job is to criticize, blah, blah, blah. When all else fails, count their spelling, grammar and logic errors and believe that you're smarter than these people.
2) Your promotion efforts fail, costing you time and money. A seminar on promotion also costs time and money. Think of it as a learning experience, and remember, it might have worked. Final sop to your wounded self: There are two endeavors you never hear referred to as "science" -- marketing and gambling.
3) Your incredible, high-concept idea did not light a fire under publishers, readers or the press. It wasn't good enough, or you didn't get lucky. Focus on the luck aspect, but try to do better.
4) You're not making any money. A job makes money. A vocation makes you crazy, which is why religions use the term. Go back to thinking of writing as a hobby. Hobbies are fun.
5) Your writing friends are more successful than you. Be glad someone is keeping the publishing industry alive.
6) Focus on the good stuff. I got published. I learned a lot. I'm still learning. I have the luxury of putting my ideas out there, and that's a big deal.