I am not very productive person, so take the following with a grain of salt:
If you fail, get up and keep going. Saying "I must do this 100% correctly, otherwise it is meaningless" is just setting yourself to fail... because sooner or later some random event will interrupt your progress... but it doesn't matter if you just lose one day and then continue with the original plan; assuming you actually continue (instead of "just one day" becoming your every day). In long term, perseverance wins.
Perhaps the proper way to think about this is that if you keep following your plans for ten days, and then fail on one day, those ten days still did something useful, and they also helped to establish a habit... not perfectly, but better than when you started from zero. That is, instead of feeling disappointed, you should focus on the fact that the second start will probably be easier than the first one (unless you needlessly make it difficult for yourself by overly focusing on the failure).
If you want to have multiple good habits, it is probably easier to introduce them separately, not all at the same time. (Exception: unless they are connected. For example, if one habit is "cooking at home, instead of eating at restaurants", and another habit is "eating healthy meals instead of junk food", getting both with one change is easier than making two separate changes.)
Sometimes, maybe your original decision was imbalanced and needs revision. Do you want to give up reading forever? If not, did you leave enough space for reading in your schedule? Reading at night is a bad habit, but having an "unproductive" day once in a while is necessary to recharge your batteries.
The "downward spirals of further failures" are a self-fulfilling prophecy, if you fear them so much that you never allow yourself a break, and then you burn out. You need to find a sustainable pace. And that pace may increase in the future... gradually. For now, focus on the fact that your average day in 2021 is quite productive, and that whatever you did once, you can do again.