Hey Jimbo, sounds like you’ve been doing some reflecting! Here are a couple of my thoughts:
Do you use scripts? Maybe this one can help you get out of the loop. It makes a guess on what word you might have confused it with, and puts a couple options right on the screen for you, so you don’t have to wait until both end up in apprentice.
[Userscript] ConfusionGuesser
How about setting aside some time to look at the items you got wrong on the summary page? The following script keeps track of all wrong answers until you delete the history. You could have a look at your troubling items at the end of the day, instead of just during reviews.
[Userscript] Wrong of the Day
Me neither 
. My main inspiration was just to challenge myself at a time where I wasn’t being challenged at all. I watched a Japanese movie, and the love for the language ignited. Now I continue mostly because it’s still challenging, and because I managed to find a good group of people to practice with. My social network was not so big before I started. I also managed to make a couple good Japanese friends, and meet my boyfriend through learning the language.
It would seem you are right at that (upper) intermediate level where it’s hard to see progress. With Minna you’re mostly learning grammar points that look the same as previous ones, just with a slightly different nuance. It’s all very formal and bookish, and unless you start reading newspapers and professional essays, good luck encountering some of them in the wild.
You say you have a Japanese girlfriend. How about you start texting to her in Japanese? The bar might be a little lower than speaking, because you have time to think about the grammar. It might add another layer to your relationship if she feels free to express her thoughts in her native language to you. I’m not sure how serious you are about her, but being able to speak with her family, might help as well. I’m assuming she and her family live in Japan and speak mostly Japanese at home.
I’ll leave it at this for now.