You have to get into the mindset that they need you more than you need them. If not, you walk. Make them Believe it is so. - @practicalthought
I agree with that!
I might add - The steps prior to that is critical:
Your sense of self-worth and frequent self-assessment and inventory.
What is that?
The exact things that are in your rant here, but more methodical and proactive - do this regularly and not wait till you are in an emotional state like this.
A classic and must read book -
How to Sell Yourself by Joe Girard
A pdf summary:
http://www.educationalservice.net/ebooks/ebooks-001/eseb20090503_HOW-TO-SELL%20.pdf
All the best!