how to discipline yourself to study something mentally challenging?

First off you need to have an interest in something you’re going to learn, because it’ll help you keep going and help understand better. If the topic is something that doesn’t intrest you try to make it personal or just persevere because sometimes when first impressions (of content) can change once you really get into it.

I am not good with long answers but it boils down to discipline, time and certainly money to be able to do that. For me, I also need an alarm to remind me it’s time to study