How to be efficient as a graduate student juggling classes, TA and research?
When your work is way too difficult to achieve your goal of the day in your allotted time


... get used to it. If you are an interesting person doing interesting things, there is never going to be enough time for all the things you want to do. And as soon as the work you're doing now starts to get easier for you, you'll get new, more challenging responsibilities :)

You won't be able to manage your time more effectively until you get a little better at estimating how long things will take, so start there. It's pretty common to underestimate how long things will take - try quadrupling your initial estimate, and see where that gets you.

When you can judge more accurately how long things will take, you will be in a better position to manage your commitments. Once you realize that your homework takes you 8 hours, not 2, you will be able to plan your week accordingly, without feeling guilty when you invariably have to drop something else in order to do your homework properly.

In the meantime, 


  Keep forgiving yourself and keep working


That's from this answer by Piotr Migdal, which I highly recommend you read.


  I miss dinner, I sleep late, and the efficiency just plummets through the week.


You don't need me to tell you the answer to this one. Take care of yourself. For more details, see How to prevent physical/psychological health side effects of workaholism in academia and research?.


  do you switch work, or do you keep going?


It helps to have tasks you can work on when you are low on intellectual energy, but that are still more productive than staring at reddit. Here's a list of ways to procrastinate productively. 

Ultimately, though, if you need to take a break to recharge, just do it - then you'll get back to work and be much more effective.