Something that's worked really well for me has been
replacing any negative thoughts with a positive alternative. The only way to effectively break a habit is to replace it with something else, so if a negative thought would come into my head, say "Argh, this painting is going to suck!" I immediately replace it with a positive thought about that thing like "Well even if it does suck at least it's good practice!".
It took about a month of doing that constantly for it to become a habit for me, now my head automatically jumps to a positive thought if a negative one pops in, so it's definately worth doing
