1700 on Lichess is good, but not amazing - it puts you in the top 45% of people who play blitz on Lichess. It's the sort of platform from which you can decide to dedicate time, effort, and hardship, to excel and maybe aim for a title one day, but it's also still very reasonable to chill out there and just enjoy yourself.
If you're not looking to reach for those heights yet, or at all, don't focus on the rating points. Play blitz until you're not having fun, or you need to go do the washing up/go to work. If you are winning and having fun, the points you're gaining aren't what matters. If you're losing and having fun, the points you're losing are irrelevant.
With that out of the way, I have personally found that it's mostly familiarity and sleep. For a period, I completely shirked bunch of responsibilities (don't do that) and played the heck outta bullet, used the "analyse your mistakes" feature on lichess, looked up openings, and slept whenever I felt tired, with no alarm to wake me up. During this time, not only did I have a great deal of fun, my rating went up quite a bit, and I think I improved (as my rating stayed there).
Now I've been playing mostly atomic for several months, and not spending nearly as much time playing standard chess, and sleeping in such a way to line up with a full time job which very much destroys me. My standard play feels very much like you describe your "brain fog".
So long as you've having fun, just stick to it, and make sure you're sleeping right. It's also worth making sure you're eating right, too, but I have no experience with poor diet's effect on my psychology 'cause I always eat reasonably well.