Related to what Kim Johnson said- I find it helpful to 'collect' supportive phrases so I can let myself off the hook when negative thoughts come in. So if the negative thought is "Geez I'm an idiot" I think about it for a bit and then rebut it with something like "well I could not have been expected to know that" or "its easy to make mistakes the first time" or "anyone could have messed that one up" That way you're not denying yourself your feelings- you FEEL like an idiot, and that's ok, but you acknowledge that in fact, no one is perfect, you're no worse than anyone else, so it remains a feeling, and not an absolute truth.
Another thing I do when I am stressed is to write down everything that is stressing me out on a big list. Often I come up with 10 or 12 things, and when that's done I see it in perspective: Geez NO WONDER i forgot to take the rubbish out tonight- I'm putting up with a of stress lot in my life right now! Often the list makes me realise what good coping skills I do have, and that helps me to forgive a smaller thing.
hope it helps you!
MissE