That's a set expression meaning "I don't care about your businesses at the moment". You can also say something like "Мне сейчас не до русского", which would mean that you don't feel like studying Russian now because you might get tired of it, for example. Or "Мне сейчас не до шуток", which would mean you don't feel like fooling around now and want to be serious. Or "Мне не до твоих проблем сейчас" would mean that you don't feel like getting immersed yourself in someone's problems because you probably have your own ones. So... It's just a set expressions.

This means something along the lines "I have more important stuff to do", "I can't be dealing with you/this now". The expression can also be applied to inanimate objects: "Мне не до работы сейчас"

нет времени для тебя

That means that someone is not in the right mood to talk to someone else.This phrase is usually said when someone is busy or just isn't ready to discuss some problems, for example, a parent can say that to their child or spouses can say that to each other.

"Мне не до тебя" means something like "I do not want your company", perhaps because I'm not in the mood for you, I'm busy or you've annoyed me.