Ha, I feel you! It took me a year to get into a rhythm that was productive and yet not draconian and is most importantly, sustainable. I'll share it with you. It's a combination of every productivity article, book and video ever, plus trial and error, and what works for me (I'm a morning person, and my energy sags in the early afternoon).
1. Don't ever write a to do list, but create a timetable in which you assign tasks based on priority, difficulty and energy. Difficult gets done first, as it's generally also important and you feel amazing to get it completed, and successfully. Priority gets slotted in during the morning so if I miss my deadline, I have time available in the afternoon. Energy means do cerebral things early, and leave emails and invoices, etc until later. They require less of you, so do them when you have less to give. I do this daily, the night before.
2. Create accountability. For me, this is having a WBF (work best friend) with whom I share what I must do, she does the same, and we shame or guilt each other into doing it. Public declaration of intent generally increases possibility of execution.
3. I write down the purpose that drives every task. That way if my focus is drifting, I can easily pull myself back into it because I remind myself not what I need to do, but why it matters.
4. The rule that if you can get it done in 2 minutes, do it, is genius. You do everything faster, and those silly little "it can wait" things actually do get done.
5. I start each day with a conversation with a human being (emoji and texts do not count) whom I trust, like, love or respect and I find that sets me in the right frame of mind. Plus often gives me new business ideas or a way or introducing networking into my day without it seeming craven.
6. I allow myself a guilty treat every couple of hours. Shamefully, for me, it's playing Words With Friends, but I never claimed to be perfect, did I?
Good luck. And thanks for asking the question, because I look forward to learning from other answers. OK, now I'm off back to work.