Best ways to accept being not always productive?

I don't think there's a catchall solution but if you break down your guilt of not being productive, there may be some underlying themes.

1) Good chance you're guilty about not beind productive because you've seen other people launching products or releasing progress shots. Realise it's inhuman to be productive 100% of the time, everyone who's released a product or those screenshots have also be unproductive at times. Refocus your goals, make them feasible and iterate on the tasks, don't tackle it in an all-or-nothing manner.

2) Look at what you have achieved and don't underestimate it. There could be a chance you're overweighting the remaining tasks and underweighting patting yourself on the back. Positive spirals exist just as much as negative spirals.

3) What is productiveness for you? Walking your dog is productive, just not directly useful for coding a website (although you could be generating ideas during the walk).

Maybe you feel like you should be doing something constantly but if you take a step back, what race are you actually running in? There isn't really someone else on the track, you're just imagining there is and accepting that this race doesn't exist outside of your mind might make it easier to appreciate you don't have to be constantly working and burning out.

You shouldn't attempt to always be "switched on" as that is usually counter productive. Just measure what your current production level is, write it down, and make an effort to stay consistent or improve. That way when you set your goals for the day, you know what you can comfortably achieve, and when you acheive it, you know to stop or risk it affecting your next day negatively.