What coding language was the most difficult to learn and why?

Sort of a hard question to answer. Some reasons why it might vary a lot:

1. Your earlier few languages might be harder to learn. As you learn more programming languages, learning yet-another language becomes easier. The syntax can be similar, and you know what pitfalls to look for. For example after learning Java, learning C# is a lot easier than doing it from scratch.
2. You may learn a lot *more* of one language. For instance if you spend years coding Java, you might have to deal with all sorts of difficult rare edge cases. If you spend only a few months writing quick/small scripts in Python, it might not be a fair comparison. (Though arguably Python is easier to learn than Java in this case, so bad example on my part :P)
3. If you have a good mentor at some point in time for a specific language, it can make it drastically easier for you to learn. For instance I spent a lot less time floundering around and failing while learning Go, simply because I could lean on someone on my team who was really good at it already.

For me personally, the hardest thing I actually know how to do was learning C programming, specifically in the context of embedded programming. It's sort of a cop-out/fake answer (sorry), because the language itself wasn't all of the hardness - a lot of the difficulty came from having to read massive device specifications, dealing with weird bit settings on registers, and other things that aren't really a property of the language itself. C also makes it really easy to shoot yourself in the foot (and face), compared to a lot of other languages.

(Don't get me wrong, it's easy to shoot yourself in the foot with basically all languages, just a lot easier to do it in C).

I'd say the hardest "language" to pick up really isn't so much the issue anymore. Only people working on very specific projects use anything like an assembly language anymore. I would say that since you are learning Java, which is an object oriented programming language, that should serve you well in the future. After all, it is an understanding of the concepts that is the hard part. Otherwise, you are just splitting hairs with syntax.

In a computer science degree program, you will most likely be exposed to a handful of languages, so exposure shouldn't be a problem for you. I wouldn't worry about trying to nail down any specific language until you have a better idea of what you will be doing in the future. I work as a Data Scientist, so for me it is all about Python, R, and SQL. But my exposure to JavaScript, HTML, and Java in college has definitely come in handy over the years.