I want to say "it was a rough year", but I'm not sure how to put it.
I thought of "難しい年頃" which I understand means "difficult age", normally used to describe the rebel age of kids.
How do people normally put it? 難しい年? Can we use 大変な年？
The main point for saying "rough [time]" is that you should express [time] as a proper time period.
(Cf., 良い一日を "Have a nice day".)
大変な一日だった／でした is I think common for "I had a rough day". 大変な一年 works similarly.
難しい一年 works as well, although it's more like "difficult year" than "rough year". That is, 難しい一年 sounds a bit more collected, as though you were in control (at least emotionally), in spite of hardships.
Earthliŋ has already provided a great answer, so instead of repeating what they've got, I figure I'll just fill out the information as it relates to the phrases presented in the question -- ways of saying it without 一【いち】, basically, and how natural they may or may not be.
I did a few Google searches for various phrases (putting "quotes" around the terms to get exact results) and this is what I came up with. Google results aren't everything, naturally, but I believe that they provide a great general reference point.
To emphasize Earthliŋ's point, 「大変な一年」 gets 12.8 million results in Google, more than twice the results that 「大変な年」 gets. Applies also to 「難しい一年」. But yeah, your safest bet would be with 大変, I would say.