You sound exactly like me. I've mostly just accepted the fact that this is the best way for me to work. I can buckle down and grind out a full day of work some days but not for many days in a row.
I've come to the conclusion that I am usually as productive/more productive than those around me and I tend to think it is _because_ of the way I work rather than in spite of it. When I do sit and do some work I am less stressed and can use a couple hours of motivation to knock out a days work in 2-3 hours usually in the morning. The rest of the day might be meetings/exercise/reading/social media/youtube and perhaps an additional 30-60min of work.
Ultimately I've found that the reason not to "slack" so much is less because I am not productive enough and more because when slacking I often lean on high dopamine activites like social media/youtube etc. that ultimately make me less happy than being with friends/going for a walk/learning something new.
I will also say I have done the "grind" for months at a time and looking back I know that I would have gotten the same amount of real work done at my current pace. The issue is the added stress makes it harder for me to think, I often end up working on the wrong problem as I am trying to move too quickly, and having too many pending tasks makes it harder to make decisions/prioritize.
I've also worked with those that grind and I've noticed they don't always work on high priority tasks. Often they are too focused on grinding they might not be able to step back and realize they could use their time better.