Tell yourself you can’t and you won’t; tell yourself you can and you stand a chance.
We only use a small proportion of out brain, approx. 10%, some will say 20% others 5% the important thing is it is not much; however that does not mean our unconscious mind is not listening or taking notice of everything we do.
Our minds also only process positive actions so if I say think about pink elephants that is what springs to mind, equally if I say ‘don’t’ think about pink elephants, pink elephants are the first thing that springs to mind.
By the same process if tell myself I cannot do something I almost certainly will not do it, if, however, I tell myself I can there is every chance I either will achieve or at least will find a way.
If say I am a bad time keeper and never arrive anywhere on time then the likely hood is you never will if however you say I am a good timekeeper and arrive on time then you stand more chance of keeping appointments.
Try it, make it something simple at first like timekeeping or getting up earlier and discover that it works then go for the more complex or difficult things like losing weight or being more patient.