+118609 I think any totally new concept can be really hard. Once you have the basis for a topic then it gets much easier.
It does for me anyway.
So if you are very little division might seem impossible. 
+1253 I still remember when I was maybe 3 years old and I had a babysitter who would teach me basic math like 6+8, 5-3, etc.
She told me about multiplication, and I remember saying "I'm never going to understand that!"
If you find something hard at first, when you attack it and learn it, you'll get it!
I honestly struggle with fractions and graphs, but it all depends on the person. If a particular topic interests you, then you would probably put more effort into learning it.
Have a great day!
+1253 Well, if you're younger, learning about variables can be really tough.
Geometry and proofs with memorizing theorems is also challenging.
Learning about integrals, derivatives, that stuff in pre-calc is pretty hard (i'm learning this stuff right now)
Depends on your skill level.
