Hello @thomasmalcolm and huge WELCOME TO THE FORUM first.
It’s like I’d read my own story so I’ll say nothing more but just give you the link to my topic Tatjana - progress reports. It contains 493 posts. I believe my frustrations will not help you as I’ve gone through many of them, accusing myself not learning enough, not being perfect enough, learning too long time having nothing rememberred etc, etc, but what WILL HUGELY HELP YOU I believe are the answers and encouraging posts many members of this forum gave me. And … maybe you need the same thing I needed - turn the mind a bit toward “growth mindset” not striving to perfection too much but rather celebrate what you’ve acheaved. It all is there. And yes, as @craigf says, Lesson 25 is brutal. It contains everything what you’ve learnt so far in a bit (just tinny bit) accelerated speed, so you really mustn’t be frustrated with that.
You’re doing just fine, believe me and, please, don’t drill into lessons and stuff for 3-4 hours a day especially if it isn’t particular fun to you (I know I’d be tired and even this for a bit frustrated to do something 3-4 hours a day). It is too much
(O boy … and I said something more, didn’t I? I just can’t stop “talking” … 
)