Your mind is for having ideas, not for holding them. When you try to remember everything, you forget things and your mind becomes cluttered with reminders instead of free for thinking.
Always have a capture system with you. This might be a small notebook, a notes app on your phone, or voice memos. The format does not matter. What matters is that you can quickly capture anything that comes to mind.
When an idea appears, capture it immediately. Do not tell yourself you will remember it later. You will not. Write it down. Then forget about it. Your capture system will remember it for you.
Set up your capture system today. Keep it with you always. Then practice capturing ideas as they come. Notice how much clearer your mind becomes when it does not need to hold everything.