"I broke up with my boyfriend last month."
It's a normal "Wakaremashita", so I reported that audio... it really sounds like "wagna" xD
Would it be wrong to specify 先月に?
Also, I'm tired of some kanji input not being accepted. It's much easier to remember 分, since it means part, or split, than it is to remember わ, which doesn't hold intrinsic meaning.
Don't use the に after words like 先月、今月、and 来月, because these are set amounts of time.
に can mean in, at, and to. So if you were to put に in the sentence, it would be as in. We would not use in like this in English either.
Ｘ In last month I broke up. I broke up in last month.
Ｏ Last month I broke up. I broke up last month.
You can use it after 月 in other examples.
O ５月にわかれた。I broke up in May.
The kanji for breakups would be 別れる. I am not sure if that Kanji is in the list of the first 100 that they use on Duo.
分かれる is for separating things or space.
I agree though, having Kanji makes it easier to read.