I've had no trouble learning the basics of a relational databases. The problem is that the basics don't really transfer between implementations all that conveniently.
It's like how I know the basics of processor architectures. There's registers and caches and interrupts. But now ask me to apply that to a specific machine's code? Now I've got to read the whole manual for that machine to figure out how the 'basics' apply to this particular machine.
Everyone has their stumbling blocks, so I'm not judging you - only that I think it's not that hard for myself and an essential skill for mostly every app developer except in niche industries.
But I guess I'm curious what your stumbling block is. Is the database servers themselves, or is it something like SQL that gets you? SQL was odd to me when I first looked at it due to its declarative nature, and having to think in sets. But it was quite intuitive once I used it for a little bit. Indexes... I knew about them from when I was learning about C++ data structures, so it should be a transferrable skill.
SQL is probably only 1/50th of what a modern database does.
What is the proper method for returning a result set from a stored procedure? Is that a basic DB skill or not? Because that's the kind of thing I end up needing to know on day 2 of the (conceptually simple) queries that I need to implement. "Oh, this query didn't work well? I've done a pseudo-close?"
The problem is that databases solve one very constrained subset of data storage needs elegantly, and then throw on a mountain of hacks to get the rest of the way there. Basic database knowledge gets you as far toward modeling real data in the same manner that basic arithmetic gets you an understanding of modern physics.
It's like how I know the basics of processor architectures. There's registers and caches and interrupts. But now ask me to apply that to a specific machine's code? Now I've got to read the whole manual for that machine to figure out how the 'basics' apply to this particular machine.