First, no matter what happens, do not put the model in the trash. If your patience reaches an end, for whatever reason, put everything in a box and store it in your closet. Patience slowly recovers over time. Model skill increases with practice. One day, you may find yourself with the patience and skill to continue across the finish line. If nothing else, the model may supply spare parts for a future project.
I looked up the model. A large number of GAZ-AA trucks were used to supply Leningrad during the 1000 day siege. This often required crossing a lake and the trucks became very dirty. The larger decals are probably patriotic slogans. Russians often painted slogans on vehicles to express their solidarity and support for the war effort. Vehicles in front line combat rarely carried such slogans for obvious reasons.
Cookie Sewell states in his review of the original model release that these trucks would all have a 6 digit registration numbers on the tail gate. All other markings are optional. Assuming the decals in your model are correct (sometimes model designers get things wrong), there was another marking pattern with the vehicle number painted on the doors.
That means you have some options! If you have spare decals, you could splice together your own vehicle code. If you only have enough letters and numbers for one code, put it on the tail gate. If you can make two codes, put them on the doors.
Let’s assume you put the vehicle code on the tail gate. Now you can put patriotic slogans on the doors.
If you put the vehicle codes on the doors, a patriotic slogan can go on the tail gate.
If you are okay with weathering your truck with some mud, you can hide any decal issues under some strategically placed mud splatter. The trucks supplying Leningrad could get extremely dirty.
I would also like to address the potential wheel alignment issue. If that happens, you have some options. By slightly shaving a mounting pin or slightly enlarging a mounting hole, you can easily raise or lower a wheel by a fraction of a millimeter. A small shim will help keep the wheel stable. If things are really bad, a small terrain base will hide everything.
With regards weathering the canvas bed cover, washes can work. Oil paints can work. Airbrushing light and dark areas can work. You will want to put a slightly darker shade in recesses and a slightly lighter shade on raised areas. That is really all it takes.
When dealing with thin decals that like to fold over on themselves, I will slide the decal from the carrier directly to the target location. Wet the decal but do not remove it from the decal paper backing. If you are using a setting solution, put that on the model. Lay the decal paper backing directly on the model, next to the spot where you want the decal. Carefully slide the decal to one side until a little bit hangs off the decal paper. Get that little portion of the decal in approximately the correct position. Now slide the decal off the backing directly onto the model. Alternatively, slide the backing paper out from behind the decal. Use a clean brush to move the decal around as needed. Use that same brush to remove excess setting solution.
The method described above makes it difficult for the decal to fold over on itself.
For what it is worth, I am making a come back to model building and experiencing the exact same sorts of problems you describe with the GAZ-AA. I encourage you to keep going and do your best! Even if the model turns out far less than perfect, the lessons learned completing the project will make all future models easier. Some day, you may find yourself looking back at this project with affection.