Patience is, like everything else, a learned skill. You develop it one step at a time - but it requires you to take active control of your choices. That means recognizing the fact that the things we do, even the subtlest, most automatic habits are the results of choices we make. Either choices to take control of what we're doing, or choices to let go of that control and simply let things happen.

It won't be instantaneous, but the more you catch yourself giving up that control, and then reassert it as a result, the more in control you'll become, and the better you'll be able to control your base urges and tendencies. From there, you'll find it easier to be "patient".

Don't focus on every detail. You will get used to drawing over time, and this will in turn help you at getting better at seeing and adding details.

Try speed drawing stuff, like setting a timer to 5 minutes to draw anything, then do it again and again.

I struggle with impatience, too, both with myself and with the process. I've found that it helps me to know when to set a drawing down, and only to start drawing when I have a good long time to spend on it without any particular hurry.

Setting a timer is also a good idea, either to limit yourself to 5 minutes so you have to use that time efficiently, or setting it for say, half an hour, and knowing you have to use all that time on a very simple exercise.

I'm a little guilty of multitasking ; I'll put on an audiobook so that my ADHD brain can give the excess energy to listening instead of trying to hurry. I don't know if this is great for learning, but it's let me stick to my exercises and do them at a slower pace than I might've otherwise.