Hey there!
I'm definitely not the best person out there to answer that, as I don't have the same problem as you, but I do relate with your reading time. In my case, I'm a pretty slow reader (and my speed goes even further slower when I'm reading something that's not in my native language, that's 100% of the time when it comes to VN) and since I'm quite busy irl lately, I can afford maybe one hour each day to read. I finished DMMD in the beginning of the year and it certainly took me a bit more than a month to finish.
But I don't think that should make you unmotivated! It took me several months to finish my favorite VN and it was very worth the ride, long as it was. Take the time you need and try to think in tackling these VNs one route at a time, for example. The goal will feel a lot closer that way. FSN can be quite infodump and long winded at times, but it does a decent job at making you remember stuff with short (okay, some not that short) explanations from one route to the other. Plus, it have a "status screen" that list a lot of character's powers that can be accessed at any time in case you forget something.
And you can also take notes. Like, every time a scene ends, or you stop reading for the day, try to write down/type in a notepad file (create it inside your VN folder) what happened with a single or a few sentences. You can oversimplify the events, pick the most vital part and putting it in all caps, crack a few jokes in your explanations, anything that you think it can be fun to do and easy to associate with the whole scene, so it can "trigger" your memory of the rest of scene without you having to read it again.
If you're feeling a bit overwhelmed to start a very long VN, you can always try a shorter one first until you're confident enough with a method to help you remember the story.
Sorry for the wall of text and best of luck!~