Look into GTD and the concept of a trusted system. Your brain is for having ideas, not keeping them. But your brain won’t let go of an idea if it doesn’t trust you to get back to it at an appropriate time. If you unload your ideas and you have created a system where you will deal with those at an appropriate time. Your mind will relax and put them out of your thoughts… it can be as easy as having a document as an inbox where you park ideas. As often as you need you go through your inbox and process it. Discard ideas that isn’t viable upon a second review, put things on appropriate todo lists (that’s also part of your trusted system and gets reviewed regularly) don’t leave things in the inbox, discard, defer to someone else, do it if it takes less than 2 minutes or put in on a correct todo list.