Good on you for acknowledging that you want to change this about yourself. As the posters above have said, admiting it is the first step, asking for help is the second.

I've been through a really tough time in the last 18 months, for the first 12 or so months I was so bogged down by the negatives, I was forever moaning to my friends (geeze I'm amazed they stuck around I was such a negative nelly), not ever being able to see the positive, but I kicked myself up the butt one day and pretty much gave myself a HUGE talking to.

These are mechanisms that have worked and are working for me

1. A journal that I write in every day - listing all that I don't like about myself, like in my life - even little annoying things about myself
2. A list of things that make me happy, be it memories, songs (the songs are good to pull out when you're feeling down - start singing a song in your head that makes you smile)
3. A gratitude list, things I'm grateful for, that I am blessed to have in my life (this morning I was grateful for freshly brewed coffee & bread still warm from the bakers)
4. The biggest one is to look objectively at every negative situation and try to see what I have learnt from it, or something positive from it. Example - went for a high paid manager's job (6 month assignment), didn't get the job, yes I wallowed in the negatives for a few hours but then I started to look for positives, that I didn't have to supervise staff, that I wasn't tied down for the next 6 months, that I wrote an application that got me an interview, that I now had a list of things to work on professionally before I go for a job like that in the future.

Hope these help.