How do I stop blowing 'won' games & 'chess slumps?'

Three out of my last four rated games I've blown positions in the middle game that the engine says I have a decisive advantage: + 2.9, +14 & +15 in my favor. It's been frustrating, obviously. At this point, it feels like it's all an inside job where I get tight & I stop playing good chess because I'm waiting for 'it' to happen again.

I also just played an unrated game 10 | 5 vs. a 1395 rated opponent & won. WTF?! I've never beaten someone that high before. These past 3 blown games have been vs. ~1025 opponents. I get a decisive advantage, become unsure about how to proceed with a plan & self-destruct.

What advice do you have about blowing 'won' games & 'chess slumps?' This certainly feels like a miserable slump!

I'm including some of these blown games below for reference. First one, playing as black: Lost sight of a simple back rank vulnerability. +14 to mate in one move.