Dear me,
Please stop and take a few deep breaths. Please stop feeling guilty about saying "no" to people or commitments you don't care about and please stop focusing on the never-ending to-do list in your head.
You deserve so much more than to feel crushed under the weight of 20 college credits this semester. You deserve to focus on what makes you happy and to not worry about other people all of the time. And, you deserve to focus on what will help you grow as a person instead of performing the daily mundane tasks you hate.
Stop using the "one-day-at-a-time" or "one-meal-at-a-time" thought process of getting through the day. You know you have your entire life planned out and if it's going to feel like you're taking it "one day at a time" for the next two years--let alone 50 years--you're never going to be happy.
Dumb things happen in life, life gets busy and sometimes you just have too much to do and not enough hours in the day. That's life. But, you can stop pushing yourself into that corner. You can take back the pieces of yourself you've given to other people. They only have those pieces in the first place because you don't know how to say "no." You can be happy and find joy in every day.
No, you're probably never going to stop hating your music theory and ear training assignments. And you're still probably going to hate the 18-20-hour days. But, you can choose to focus your efforts on things you are passionate about.
It's not fair to you and it's not fair to other people when you don't want to be there. Stop wasting your time and start using it to better yourself, make yourself happy and enjoy the parts of the day you love. It's not worth it to be so stressed out to the point where don't even have time to relax without constantly going over your to-do list.
It's not fair to over-extend yourself for other people. It only makes you miserable and leaves the other person working harder because you can't totally dedicate yourself to the task at hand.
This is nothing against you, you simply want to help other people all of the time. But, sometimes you really need the time for yourself. As the saying goes, "You can't pour yourself out to people if your glass is empty." And it's true. To be a better version of yourself, stop and take a breather. Write everything down so that it's not swirling around in your head. Make sure you drink enough water and then focus on what makes you happy.
By sharing with others your joys and your ambitions, you will find people who have the same life goals as you. By working with them and helping them instead of doing everyone else's mundane tasks, you will be happier and it will allow you to be able to grow yourself instead of drawing yourself down.
Sincerely,
The over-extended and overwhelmed bullet-proof tiger