I think this gets the causality wrong. I'd say you tell yourself negative things about you because you're already in a negative mindset. Similarly you're more likely to think positive thoughts when you are happy.
So I'd say rather than simply saying nice things, one should be conscious of when one is thinking negative thoughts and then try to see where they come from and then what can be done about it. Being honest with yourself is a big part of that and I think that's what many people have a hard time with. And of course: If you're happy, also try to notice and cherish that. (If you look in the discussion I've had with another poster in this comment-thread, we've talked a bit about this too)
So I'd say rather than simply saying nice things, one should be conscious of when one is thinking negative thoughts and then try to see where they come from and then what can be done about it. Being honest with yourself is a big part of that and I think that's what many people have a hard time with. And of course: If you're happy, also try to notice and cherish that. (If you look in the discussion I've had with another poster in this comment-thread, we've talked a bit about this too)