I'm an absolutely -terrible- language learner, mostly because I have a terrible memory. I managed to fail a first semester course in Spanish once, supposedly an 'easy' language. (Knowing more about language learning now I have my doubts that languages differ that much in 'ease' unless they are very closely related to one's native language.)
Nice thing I've found out about that though is, memory exercises improve your ability to use your memory... so the more I've used SRS tools the stronger my ability to recall facts becomes. My ability to retain facts or lists or the like is still pretty bad, but definitely better than it used to be, and of course my ability to retain vocabulary too.
Any-way, I more or less persisted for for years. I did have about a 2 year lapse and several other months long lapses in my studies, which set me back pretty far each time (especially the 2-year lapse! I was shocked one day when I couldn't read some kana, and that motivated me to resume my learning. I kind of had to re-learn everything but not entirely... most of it came back pretty swiftly.)
Anyway, simply by keeping at it I've managed to reach the point where I can watch a lot of shows with 'everyday' vocabulary with full understanding (the more technical, philosophical, or political a show gets the more it gets into vocabulary I either don't know or only know to read not to hear). I can read just about any fiction now - manga or prose - with just a little help from the dictionary. I passed the JLPT N1 test almost a year ago now.
As far as motivation issues go, I've had experience with 2 different kinds of motivation problems - the first being that just studying get just get so tedious. It stops being fun. Solution to this, for me, was to engage with native material (particularly an old CLAMP manga called Magical Knight Rayearth, but that's another story). The passages I found that gave me trouble then motivated me to want to study more, and going back to studying suddenly wasn't 'tedious' because it was searching out the key to understanding. When fed up with studying again, back to native material.
The other motivation issue I've had trouble with is simply ... fear of failure. If I do my flashcards I might get them -all wrong-. If I attempt these chapter exercises in my book I might get them -all wrong-! Of course I wanted to learn, so I would get to it -eventually-, but it did feed into a lot of procrastination where I did things that weren't actually useful ... too much time reading and writing in English on sites like this one included, although a -certain- amount of that is useful for motivation and picking up new learning methods. Once you get into the off-topic conversations, accounts of 'what I did when I was in Japan', debates about various Japanese laws or how they treat foreigners, pictures of signs with bad English translations, etc, etc... you should start to consider that you may be procrastinating and not actually learning about learning anymore.

I still don't have a good solution to this problem. I stopped procrastinating about Japanese because I stopped being afraid of failing once I could easily read easier manga. I was confident at that point that continuing on would make more challenging manga, novels, etc., -also- easy to read. With confidence, I had no need to procrastinate.
Unfortunately, I still procrastinate in other areas of my life where I have less confidence, so, ehhh. All I can say there is that you -should- have confidence, if you keep at it, understanding will come, as long as you have a good mix of native materials and study materials that you work with. By -not- procrastinating it will come faster.