The short answer is: hard as it may seem, remember that we have all been there. You will be shocked to think back in a few years how little you knew at the very beginning. It's normal. You will catch on. Just keep in mind that your supervisor gave you the opportunity, so they think you are capable of succeeding.
As for overworking: it seems to me to have a root in perfectionism. I like the idea of countering perfectionism with perfectionism: once you realize that the situation is not sustainable, you know that it isn't ideal, and you need to change something to stay productive in the long run. It seems you have already figured that out. I recommend doing some reading on productivity (I really benefitted from reading How to be a happy academic by Clark and Sousa). Start by implementing small changes right now, e.g., take an hour every day for playing your favorite instrument, make a strict rule not to work on Sundays, tell people close to you about this so they assist you with it, etc.
For more theory on how to stay sane in life in general, you may want to reflect on the eight ways to well-being.
Finally, if you compare yourself to others, don't just look at the "rock stars" for which everything seems to work out easily. More common are situations like in this question, or in this question.