My only experience is I was completely convinced I would have a lucid dream, and sure enough, I did. Shortly after it began, I became aware, but didn't have control. I don't remember what made me believe that, (whatever it was, I remember it not working afterwards) but it shows there's power behind the mind-over-matter concept.

Mantras don't work for everyone. The brain can be stubborn when you want to implant new ideas into it. It's not automatically suggestible. It'd be like watching a commercial several times, hoping the message of buying this product would eventually seep in. There's more to know about it than I can summarize, but in short, some people are more easily suggestible than others. Hypnosis is a nice method to help with MILD or lucid dreams overall with a few applications. But I'd say if MILD hasn't worked after all this time, then to definitely pursue other methods.