I fully agree, being in the right mindset helps a lot, maybe try a few games as a guest before you play, prepare your mind, so to speak.
Why am I so streaky


Well as you win you start facing tougher opponents... Then there's potential for overconfidence, or complacency... Related to mindset as you noted. There's also being more tired than you thought, due to real life issues, and just plain coming up against dangerous openings you weren't aware of... It seems a pretty common phenomena, from the number of posts you see here about it. btw #metoo.

For me, it seems to involve focus and paying attention. I catch myself making dumb mistakes when my mind is drifting on autopilot.

Mindset plays a huge factor. Paradoxically, how much you are affected by it, is also based on mindset.
Try to focus less on winning or losing, but instead on playing the best move.
In this game, move 19. So many playable options, with 11 minutes on the clock. Why the rush? Take your time to evaluate the position. It will reduces your downswings.
In this game. You resign on move 6. Even though you have 5 minutes on the clock. Why? Which mindset makes that a good idea?
Final example. Move 11. You have 4.40 on the clock. Even with an increment, it means you have not spent more than a few seconds on each move. On move 11 you blunder big time. There were plenty of good moves and you had all the time you needed, you just rushed your decision. I understand resigning in this case. But at your rating you really shouldn't. You'd be surprised how many lost positions are winnable at this level.
Keep in mind that chess doesn't care about your mindset. It's about finding the best move. You mentioned your mindset fluctuates but raw skill doesn't. Independent of your mindset, you always have your raw skill to help you find the best move. It doesn't need mindset, all you need is to spend some time to consider your options. If you can focus, great. If you can't, you'll have to fake your focus for a while.
Anyway, good luck!
I feel like chess is very dependent on mindset, which unlike raw skill can vary a lot from one day to the next.