*Note: I am actually bipolar, diagnosed in 1997, so I know of which I speak. No disrespect intended to anyone suffering from mental illness. I'm right there with you.
It all started a few days ago, when I decided I was ready to begin querying again. I sent out a few feelers into the Wilds of the Publishing Industry. I spent that evening riding on a cloud of wishful dreaminess. (If you are bipolar, you will know this is a real thing.)
By the following morning, I'd convinced myself I'd made a huge error. What was I thinking? I'm not ready for my baby to meet Publishing Professionals! I'm not good enough! My words lack substance! My characters are underdeveloped! My plot is contrived and silly!
And then I remembered to breathe. The extra oxygen helped. I calmed down enough to remember that my critique partners like my story. We've edited it into a tight, cohesive narrative. It is a Good Story, darn it!
When I recovered from my Stuart Smalley moment (People Like Me!), I had a grip on the proper perspective again. My writing is fine. Agents and publishers will either like it, or they won't. I can't control what other people like, but I can keep putting out the best writing I'm capable of. That's all I can do.
Rejections aren't a personal attack, but simply the opinion of one person. Likewise, requests for manuscripts aren't an offer of representation, or a contract to publish. I'm doing my best to remember this. For me, it's just as dangerous to get overly excited about acceptance in any form as it is to get too wallowy over rejection (however nicely worded it may be).
I strive for a happy zen medium. I'm going to let the requests and rejections alike wash over me like water over a pebble in a stream.