The answer is very simple: don't keep them in your mind.
People can remember about 7 different pieces of information at a time, so when you try to remember more things or for a longer period of time, you usually have to revisit them mentally just so you make sure that you haven't forgotten some. And this takes a mental toll, makes you tired, and distracted.
In my day time job I help and support a few development teams, which requires me to be involved in many topics at the same time and switch between them fast. What you described happens to me also. I need to clarify something, I need to send an email, I need to reply to an email, I need to setup a meeting, I need to test this feature, I need to check the database records for that other feature, while I deal with one thing, something else comes up, etc.
How do I keep track of these things?
Write to disk!
I keep a backlog of things I need to do. It's just a text file where I drop things I don't want floating in my mind.
For example, while I'm verifying some database records I may remember to send an email regarding some other topic. If I try to keep that in mind and also focus on what I'm doing, it adds strain. Do that with a few more things and it becomes very tiring. So when I remember something, I add it to my backlog in the file and forget about it while focusing on what I'm doing right now.
Then, when I'm done and I get a breather to start something else, I just go to the file and pick the next topic I think it's important. Once done, I delete it from the file. Each morning I try to plan my day by organizing the items in the file and putting first the things I want to work on for that day.
This way, I don't need to remember many things because they are written down.
Then rinse and repeat.
Place the file on OneDrive, Google docs, or some place else online with easy access and backup, and you don't even have to worry about the file itself.