I knew I was going to buy an aquarium for about a year before I actually did. And I did a lot of reading leading up to it, so I avoided most of the common beginner pitfalls.
But I was still plagued by a discouraging wave of fish deaths early on because I was determined to keep
guppies. I don't know what it is about guppies, but they die on me like no other fish that I've ever kept. I brought home a lot of guppies of different types, hoping that they'd be stronger stock. But without fail, and without exception, they died on me within days.
It wasn't until I gave up on guppies that I had any success. Once I stopped buying guppies, I didn't lose a single fish for a good long time.
So despite all the research, I still had a hard beginner lesson to learn. And that lesson was that if you are going to thrive in this hobby, you need to be able to adapt. Adapt to your water conditions, adapt to the livestock that is available, adapt to your space, adapt to your budget, adapt to the problems that inevitably arise. There are times when you need to accept failure in order to succeed in a different way.