I'm assuming you mean MDF or something similar? That seems like a weird choice for a window frame, where there's a risk of condensation but I'm not familiar with (I'm assuming) America construction rules. Yes, wood filler is definitely a better choice than plaster and will be fine if it's something like MDF.

Scrap away any loose paint. Fill with wood filler, not plaster. Sand the filler, then paint using even pressure to avoid brush strokes. Avoid overloading the brush with paint for the same reason. Sand, using a fine grit sandpaper in between the coats of paint, and allow to dry. It should be perfect.

Is this something new or something you've struggled with long term? That could help to narrow down the cause.

Try some Adrian Tchaikovsky - intelligent arachnid protagonists and other wonderful, nontraditional sci-fi that always leave me feeling like I better understand the world or have something new to think about.

Or, for something happy and slice of life-ish, Becky Chamber's Wayfarer series, or Monk and Robot. Uplifting and hopeful, with none of the things you didn't enjoy.

Also, I second Murderbot. Murderbot is wonderful, and one of the few series I enjoy more as an audiobook if that's your jam.