Time management as a TA in mathematics
I would like to point out that life as a graduate student is tough, especially if you indeed want to achieve something. 40h/week working time is much less than one should normally invest in. (At least from my personal experience and my personal observation)
Things are just bumpy in the beginning. It's normal. Just keep moving.
You are not superman. Everyone has limited energy and time, you just can't do everything in the same time. You may want to give up one of the lectures that you are attending if it's not closely related to your topic.
Plan your own research, set milestones and keep progressing. 
Try to do one thing in a day. If you work on your own research during the weekends, do not think about your TA duties or lectures at all in the same time. Make a bigger chunk of time to work on one thing instead of fragmented pieces of time is a good idea for improving productivity