How do I work through exhaustion?

I have a full-time job and a side business. I want to take my side business full time, but I am having difficulty working on it after I come home because I hit a major wall from being mentally exhausted.

I have a lot of responsibilities: I am a single mom of two and I am taking an online college class.. My day job requires a lot of thinking, organizing, data, working with 100+ people, etc. My side business is a creative/education business, which requires brainpower. Maybe it’s my thoughts that are creating my tired, but how do I work through it if I feel like my thoughts are naming how I feel, which is mentally tired?

I work on my side business all weekend and I love it. It is energizing and fun, but if I schedule to work on it during the week, it rarely gets done. I genuinely need guidance on how to overcome this big obstacle of being tired.