Falling off the wagon is an inevitability of being productive. Sometimes, so much stuff is thrown at you that it becomes less important to maintain your system than to just get on and get the work done. Because you are rushing around trying to do so much stuff, taking a few seconds to capture things feels impossible and you let things slide.
The problem with letting things slide is that sooner or later you are still going to have to pick up the pieces and do the work, tasks or commitments. If you don't capture this stuff, then you are going to struggle to remember what it is you have to do and by when. This results in the feelings of stress and overwhelm returning putting you back exactly where you were before you decided to get yourself organised.
There are occasions when something comes up in the day that means all the things you planned to get done have to be postponed. That's okay. That is part of life and having a system that allows you to just postpone stuff until another day is extremely valuable in these circumstances. (Todoist is excellent at this) But, and this is the big but, you should still make sure you are capturing those commitments. Everything else can slide. Doing scheduled tasks, reviews and planning and processing, all these can be postponed to another day. But if you stop capturing your commitments and tasks, then you really are going to be in trouble. Stuff is going to be forgotten, appointments are going to be missed and deadlines will not be hit.
So, whatever you do, however you set up your system, the one thing you must get right is your capturing system. Make sure that the method you use to capture stuff is simple, quick and easy and that everything is flowing in to the same place. Scraps of paper are fine if you have a trusted system of getting those pieces of paper into your system, so is collecting stuff on your mobile. The most important element is that you capture everything. Processing and sorting can be done when you are less busy, less stressed and when you feel in the right mood to process. Just don't stop the capturing.
Falling off the wagon doesn't have to be the end of your productivity drive. Having spent the time to set up your routine tasks and projects - they will still be there when you get back in to it - it would be a shame if you gave up completely. Instead by all means have a few days off, even a few weeks, but remember when you come back to it, your system will be in place and will only need a few minor tweaks to get it back working.