It’s not about doing THE best, it’s about doing YOUR best. It’s not about A*s, it’s about seeing what you can do with what you have. If you keep trying, you’ll probably surprise yourself with what your best actually is.
Use colours : it will help you to etablish priorities. You can use red for the urgent things, green for the ones you have the time to do and blue for the things you have time to do but you are planning to do that day (so if you don’t do them it’s negociable - work with the circunstances) ;
Use highlighters and highlight every task after accomplishing them. It will make you realise how productive was your day ;
Don’t overestimate the things you can do in a day because it will discourage you ;
Don’t underestimate the things you can do in a week! ;
Take the time to decorate your agenda : you will be even more motivated ;
Make a “To do this week” for the things you haven’t set a date to do.
Don’t stress if you don’t accomplish all the tasks you were supposed to do in a day and if you are not sure about the dates, use a pencil so you can erase it if you change plans.
I hope it helped you : if you have any question feel free to ask! (Don’t worry, your enhlish is great - and shuu, I’m French ;) )