Two comments.

• I would try and avoid negative self talk (weak) as it can really just reenforce guilt/shame. The classic example I learned in therapy was should statements. When you think/say I should have... It can bring alot of negativity but it's usually easy to change it to a could or would statement. I would have or could have xyz. Would/could can be a slippery slope to excuse/procrastinate things but it's okay to legitimately give your self a break when the circumstances warrant it. Brains need downtime, you don't have to be filling 100% of your day
• doing prep work isn't cheapening your willpower as long as your consistent and apply it to everything and not just the easy things. Another trick I learned is to try and break a task down into the smallest step you can do to make progress. Step one could literally be get a pad of paper, or change into work out clothes. It makes taking the next step easier and also is kinda like "Well I already have the pen and paper I might as well get writing" or "I'm changed so I might as well do something" even if it's just a walk or jog. But again the key is consistentcy.