In general, I don't think it's possible to simply change just like 'snap, decision made, done' Sure, you can make a decision to change but the change itself takes a long time to implement. Whether it be eating healthily, exercising or thinking positively. There's a road ahead with any decision.
I also think it's a little confusing (and too simple) to talk about positive v negative thoughts. The thoughts we have are our own and I don't think you should try and suppress them by labelling them negative. Besides, I don't think you even have the power to stop thoughts cropping up. What you can do though is learn to accept them for what they are; that is, neither positive nor negative but simply something that you think. You can contextualise your thoughts though; understand perhaps why you think that, where the thought came from, understanding maybe what influenced it or kicked it off. For me, I think the key thing is to not feel that there is some compulsion to act upon a thought. It can simply stay there in your head as a thought - and don't judge yourself for having certain thoughts.
No such thing as a negative thought - but there is, in any given context, a negative action. So sometimes respect and restraint needs to be exercised over your thoughts.
But I'd say go one step further and explore your thinking. Let yourself think all manner of things, indeed encourage it, just to see how you react. Test yourself, go into areas in your mind that you might not otherwise. Review your own reactions to your thoughts and try to understand why you've responded as you have. It can lead to some very revealing insights! Your own head is about the only place where you can really be free so run free, play and enjoy it!
(But then, I would say all of that. I enjoy playing in - and with - my head! I go everywhere... and I don't need drugs to do it either!!)
