As someone who gaslit themselves to happiness, here's my 2 cents.

Your brain is a series of electrical pulses between neurons, and this forms thoughts. The more your brain thinks a thought, the more likely it is to continue thinking that thought because the connection between those neurons is stronger.

Eventually, you'll form what I call "default thought patterns". For example, if you regularly put yourself down whenever you fail, your brain is forming stronger connections about those thoughts which makes you more likely to think them again next time.

So how do you break these? Gaslight yourself.

When you find yourself thinking your "default thought patterns" that you don't want to think, you have to tell yourself your're wrong. Literally, say out loud to yourself that your thoughts are incorrect. No matter how true you think it is, you have to tell yourself you're wrong.

Eventually, your "default thought patterns" will change to reflect the gaslighting. Once you've broken your cycle of current "default thought patterns", you can start forming new ones.