I think this is so true! The two aspects I've been trying to control more lately are the things my mind is exposed to and then how I react to those things.
Exposure: I've noticed that by paying less attention to all the fear mongering and politics in the news and just tuning it out, I tend to be at a much higher baseline of contentment and joy in life. That stuff really can be toxic and for me, minimizing it has been hugely helpful.
Reaction: I try to learn the lesson from the Stoics that there are plenty of things in life we don't control, but we can control how we react mentally and emotionally to the things that happen to us. By focusing on what I can control and choosing not to react to the things I cannot control, I end up in a much better place mentally. Still an ongoing challenge, but one that always seems to provide value when I practice it.