If you “insist on doing something every day” that is a strong way of saying that you want to do something every day. We don’t really abbreviate something to sth in standard writing or speech, it is best to type your words fully for more comprehensive answers :)

I speak USA English, but I don't understand. Are you using sth as an abbreviation for "something"? To insist is to beg, to ask again and again, to make it known that your request is very important. It doesn't mean to do something over and over or every day. I would like to help you more, but need some clarification, please.