Yes, it also is food, therefore to consume leftovers is not fasting…perhaps you can save the leftovers for later!
When you say “leftovers,” do you mean what you find in plastic wrap in the refrigerator, or what you find between your teeth long after you eat?
The stuff in the fridge would break the fast. The stuff between your teeth does not. One of the rules for scrupulous people about this is that only actually putting food or drink into your mouth counts to break the fast. Finding a crumb in your mouth and swallowing it does not.
No the crumbs in your teeth don’t break the fast. But If that is a problem you need to floss and brush better.
Do you have a regular confessor Priest that you can talk to about these. I understand your pain by the questions you ask, but I’d suggest if you want to enter the life Christ wants for you, it’s free of fear and anxiety. This place is not a place that will provide that for many who worry about these type of items, because there will almost always be at least one poster who feels anything is sin.
I can’t imagine having the level of concern you have and not having a spiritual director/regular confessor who can answer these questions and help you understand the role of confession and how to grow in faith.
I mean stuff stuck in my teeth, etc.
Of course that does NOT break the fast. In all the threads you open you are asking if every little thing is a sin and none of the things you have asked about have been sins. You really need to consult a priest as soon as possible otherwise you will have a miserable life dominated by scrupulousity.