If something is a let-down, it is a disappointment. The image here is that you have built up your hopes. You are looking forward to something, and when it does not occur, you feel as if you have been let down.
To get to the point.
Dont let it bother you. forget it.
This is a gambling idiom. If you let something ride, you leave it as it is or let it stand. The image is of you leaving your bet on the table instead of taking the money or changing your bet.
To do work and focus
forget the things that have pass
Distill down, or boil down, as an idiom, means to get to the essence of something, or to simplify it.
I've never heard that idiom before. Perhaps you mean DOWN AND OUT, which means that the person is at a low point in their life, that they're poor in every way and not likely to make a success at anything in the near future.
sit down come over to
The idiom "feeling down in the mouth" means feeling sad or unhappy. It refers to someone looking sorrowful or melancholic, with their mouth turned downwards indicating their emotional state.
This is an idiom meaning to narrow your focus down. It can also mean to narrow your aim and focus on one thing to hit. Picture the zero as a target and you get the idea of the idiom.
To crow about is to brag or boast about something. Let others know some news.
If someone "jumps down your throat" it means they react very angrily about something you said.