oh man shape language has so many aspects to it! I tried answering this a few times and kept finding myself overexplaining art fundamentals. So lemme try and analyze my own process a bit more and hopefully that'll dig up something more specific or helpful. please don't be afraid to ask any clarifying questions you have if my response doesn't fully cover what you were trying to get answers on (I love getting art questions but am not always the best at explaining haha)

So first of all the big thing is to be looking at TONS of other people's art. This is pretty obvious and you probably do this already but still it must be said!! I have big folders of my favorite artists' work and am often doing studies based off of them! How are other people using shapes? what parts of the body/outfit etc. are they exaggerating. It's hard to explain exactly how you do it because I think everyone has a different visual background and is pursuing different goals so there aren't any hard rules. I guess I would think about what sorts of designs speak to you and what aspects of the shape language is helping to pull that off? What do you like to see and why? (i feel like my art answer to anything is always just 'know thyself' LOL but seriously! cultivating strong personal opinions and taste is so important!)

A big part of my shape-language process is simplification! I lean more simple with my designs in general but like... narrowing in on a handful of distinct shapes/features and then letting the rest of a design take a step back can be important. having a base of simple shapes, stepping outside of them in a couple spots but returning to the simple skeleton everywhere else? Like having that really simple under sketch/gesture to follow. Working broad/simple and then fitting details into that framework. Whether that's like setting up basic blocked shapes of a landscape and then adding little whimsical clouds & whatnot or having a character silhouette of simple shapes & then deciding whether to add stripes to their shirt.

Hm yeah as I'm trying to dig into my process the unfortunate answer I'm turning up is that a lot of it (in my personal work & fanart at least) is just sort of intuition developed from really figuring out what I like to look at. Drawing lots of ugly bad shape language and trying to figure out why it doesn't look right.

which is a good transition to the second half of your question about my art education because there were a lottttt of ugly years LOL

I did major in Illustration! I think I had a lot of fundamentals figured out (in my head at least, the actual art was REALLY wack imo) but being forced to meet deadlines & just make a ton of stuff had obvious results. I have also always been pretty compulsive with my drawing (got in trouble for it a lot in middle/highschool)

I totally feel you about that post-school motivation slump though. It's so tough to stay consistent and it's something I definitely struggle with. I can't give you too much advice on drawing more regularly (trying to force a routine has never worked for me) but I would say if you're already sitting down to retrace old art & aren't feeling inspired or like using your brain I would suggest trying to study an artist you admire instead! like just going through and copying hands/eyes/faces you like. I feel like doing that can make me feel more excited about making stuff and even if it doesn't it's still helping train your eyes & hands! You're putting some new stimuli into the machine!