I had noticed it and mistakenly attributed it to the sunk cost fallacy but on reflection it's quite different from sunk costs. However, it was discovering and (as it turns out, incorrectly) generalising the sunk cost fallacy that alerted me to the effect and that genuinely helped me improve myself, so it's a happy mistake.

One thing that helped me was learning to fear the words 'might as well,' as in, 'I've already wasted most of the day so I might as well waste the rest of it,' or 'she'll never go out with me so I might as well not bother asking her,' and countless other examples. My way of dealing it is to mock my own thought processes ('Yeah, things are really bad so let's make them even worse. Nice plan, genius') and switch to a more utilitarian way of thinking ('A small chance of success is better than none,' 'Let's try and squeeze as much utility out of this as possible' etc.).

I hadn't fully grasped the extent to which I was sabotaging my own life with that one, pernicious little error.