How do I focus during exercise on days when I wake up feeling flat and tired and my head seems elsewhere?

For me, to focus during exercise no matter how tired and lazy you are, the first thing you need to remember is your main goal/reason why you want it at the very first place. Besides to keep you motivated, play a song that you love or motivates you to move.

First, you need to put all your efforts on putting your clothes on. After that, you'll feel like "okay, I'm already wearing this so I should do some exercise". Then, do something you like. Take it easy on you at that moment. If you like dancing, just dance for some minutes. Or if you like running, run for a while. If after doing that for a short time you feel more energized fo do some more intense exercise, go ahead and do it. But if nof, focus on doing that kind of exercise you like for the time you can. Don't be too hard with yourself.