It sounds like you're pretty much there already, affirmations are very similar. I guess the main difference is you're expanding on those affirmations and trying to make your mantra include words that spark you into action.
Like you said you have to discover it yourself, that's why I only put a couple of loose guidelines when it comes to actually choosing the words.
Take your time, it could come to you in a flash, or slowly over days or even weeks. Once you're thinking about it though, it will happen :-)
Cool, thanks. I'll try not to spend too much time trying to think one up. Hopefully and AHA moment will arise and I'll have the perfect one.