Sustaining focus and effort

I embark on new things whether it be a business idea or a drinking and eating protocol and I’m able to do it for a few weeks and then lose motivation and interest and lose sight of my reasons for choosing to work towards that goal.
I guess the thoughts might be: it’s too hard, it won’t make a difference, oh screw it, what’s the point, etc.
This causes me to abandon the mission and my reasons become less compelling.

I want to be able to sustain focus and motivation to actually achieve the result.

Ex. Stop over drinking
Reasons- I have more energy, no hangover, fewer regrets about what I may have said or did etc. but then I lose sight of those things when another social gathering comes up.

I’m not sure what thought to use:
T- it’s possible that I can do hard things.
F- confident
A- persist
R- achieve result- stop over drinking

But the problem is I can’t sustain that thought or thoughts that fuel actions to achieve the result

Pls. Help