Sorry for the long reply but this post has gotten me thinking about the models I have startred/worked on over time.
In general, after leaving home for college I had a hard time completing models and eventually stopped building for a couple decades.
Up till about 4-5yrs ago, I believe that the last models that I had tried to build were in college (in the mid to late 80's) and I never finished either. I think I lost one (an Airfix HMS Fearless) but may still have the other (an Airfix HMS Amazon) still packed away somewhere. The only thing not completed on it was trying to fit some Photo Etch to it. It was my 1st (and so far only) attempt at PE and I had started with one piece @ the bow but it didn't look right and I couldn't figure out how best to proceed, so I set it aside and never finished it. As such, I guess it would be 29yrs or so as a WIP. (Maybe I should try and dig it out now and use it for practice.)
Other than that, once I decides to try and get back into model building about 4-5yrs ago, I've found it hard to complete models.
I started a model of the Pre-Dreadnought Kniaz Potemkin, and got to a point where all that was left was adding the casemated gun barrels, PE and rigging, but I still wasn't sure how best to handle the PE and rigging, so I set it aside. It now sits on top of my TV/entertainment center in my living room maybe 80-90% mostly complete, with hopes of finally finishing it. (Though I also went ahead and bought a 2nd copy of the model to start over on, to try and learn from my mistake on the 1st model, so that the 1st one may end up as a practice kit for experimenting with PE and rigging).
Other than that I have one kit of an Airfix Fairey Battle that I was trying to convert into a What-If RN torpedo bomber, with folding wings and revised 2nd seat/radio operator/bombardier station. Unfortunately, being a What-If, without a set plan in mind I kind of started out free-handing some ideas on it, and eventually noticed that I had messed up the wing folds (so that they were not symetrical on both sides of the plane). That plus the fact that the radio operators seat the I scratch built looked a little out of scale, and the fact that I had messed up the paint a little (and accidentally dropped a freshly painted part into a glass of water), kind of led me to set that one aside, and I think it's probably officially no longer a WIP. I may eventually try and start over on the idea from scratch though with a new kit.
I also started a couple 1/25 Revell-Monogram Chevy Impala snap together Police car models. One was going to be built straight out of the box as a police car, with alternate decals, while for the other I began modifying into a What-If modern sedan. The police car is mostly done, though needs some paint rework before adding the decals, but the what-if eventually became overcome by events, and probably won't ever be finished, which also led me to stop wotk on the Police car as well (since they were meant to show a side by side comparison of the changes I did).
Additionally, I also started modifying a Monogram 1958 Thunderbird model into a what-if Sci-Fi type hover car, but as with a lot of my what-if ideas I found that part way through I had kind of messed up something somewhere, so that it was a little lopsided and asymetrical, and so I kind of decided to stop work on it, so its probably a dead kit as well.
I also have a small 1/72 scale German Boxer wheeled APC that I messed up so badly that I started trying to convert to some sort of Sci-Fi what if Lander or Hover Tank, but have set aside until I can figure out the details of what to do next on it.
I do however have a couple different WIP's under way right now that I am progressing slowly on. In general I like to do a little on each concurrently so that I don't get too frustrated with progress on any 1 model. These include converting the Lindberg Q-Ship into a WWI Tramp Steamer (maybe 65-70% complete), converting the 1/888 scale Lindberg USS Shangri-La into a modern What-If ESSEX class replacement (maybe about 40-50% complete), and converting a Revell Easy-Build/My 1st Model 2015 Ford Mustang into a What-If next gen sedan (20-30% complete).
I don't know if that means I'm having trouble completing stuff or if it just means that I kind of enjoy messing around with ideas and experimenting more than actually completing anything.
PS. I do actually have some completed recent models including a What-If concept for a modified AAV7 APC, but I think that the majority of stuff I have right now is either falls into the WIP or "not started yet" category.