To change a bad habit (which would include negative self talk) takes 21 weeks... and that is if you replace it with a good habit (such as positive self talk.)
We have an advantage though when dealing with self talk and thoughts. It takes 30 seconds for a thought to become fixed in the brain. The brain can only think one thought at a time. That means if you say something negative (or think it) you have 30 seconds to make it into a positive thought ...which said positive thought would become fixed or stabilized after 30 seconds.
If you don't dwell upon the positive thought for 30 seconds, but move on to another thought, and maybe even a negative one (such as, I said that but I don't believe it) then you don't reap the benefit of the positive thought.
It's the right way to change thinking, but correcting and countering anything negative with a positive.

It does work over time.
It does seem to take longer to remove a negative frame of mind over any one issue, than to embrace the negative....but that's because most of our negative thoughts are from habitual assault, whether by our own selves or from others.
I'm sorry you aren't feeling worthy today... but you are!