Hi Indie,
There are a lot of reasons people struggle with motivation: procrastination, lack of interest, schedules, illness, etc. It's finding what works for you that counts no matter the reason. It's good you are trying to figure out now what will work for you to help get the work done.
A lot of my lack of motivation results in being a terrible procrastinator for most things, if I let myself get away with it. I had to learn to break things down in to smaller tasks that would not seem not so overwhelming or could hold my attention for 5-15 minute increments, and then take breaks to do something interesting in between as a reward for getting through it; then go back, and have a another reward. It then became a goal, if I could just do this part of whatever assignment, then I could go do whatever else that was of interest for 5-10 minutes. Go back to work on something different each time, if it helps. It gave me some traction and it made a big difference in how the little chunks added up, and then the next thing I knew, it would be done.
I don't know if something like this might help but it is what I still have to use to this day for the things I dread doing for whatever the reason.
It's frustrating to feel like you're not able to accomplish something that you know is within your reach, this IS within yours! It's just finding the right tools whatever they may be to get you there. I really wish I had more for suggestions for you but being one who struggles with this too at times even now, I do understand your frustration more than I can say and perhaps this might work for you too. Hang in there, you CAN do it!!!!