Nixie, I can relate. My mum has always been pretty negative about things - she said she praised kids a lot when we were little but I don't remember. (Excessive or inappropriate) praise can sometimes lead to problems too.

Maybe it's good to realize that your way of thinking has benefits too - just 'positive' thinkers can sometimes overlook important info or may go about things too carelessly, in comparison to 'negative thinking'. There's even a book about the benefits of negative thinking

Aside from that, you can read books on empowerment/self-help, write down your good characteristics and accomplishments... The above advice is really good too...