JQS is right: It has to come from within. I always thought it was bullshit, the notion that I could make myself feel happy when I just felt miserable. Despite different people telling me it was possible to change how I feel, I fought it for years. Eventually, I decided to just try it. I decided one day that I wasn't going to be bothered by something (I forget what it was now, but it seemed like a big deal at the time). Almost overnight, the negative feelings vanished.
Ultimately, you have to stop and decide whether you're going to accept this new life and start to heal from the pain, or give up and walk away. One thing you can't do is spend the rest of your life wallowing in hurt.
I acutally learned a lot of this process from Deepak Chopra's 7 Steps of Spiritual Success. For me, it was enough to read the abridged version. I haven't read it, but The Secret apparently has the same message. They're both a bit cheesy and new agey, but it seems to work for a lot of people. The bottom line is that you have to decide that you're going to be happy, and then just think of yourself as being happy, and the happiness automatically comes. If you think of yourself as sad and miserable, then it reinforces the sadness.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker