I must point out before explaining my situation that I'm an English learner. However, for this question I need the help of an expert; that's why I'm asking here and not in ELL.
When writing an informal letter, I sometimes tend to use the em rule as follows:
Here I present an idea — but this is an opposite one, though. (1)
Here comes another idea — and something (closely or loosely) related. (2)
Examples of both constructions:
(1): "I hope I will overcome my father's death — it won't be easy, though."
(2): "I've found a purse — it must be Kathy's."
Is this practice correct at all? I ask because I was told it isn't: that the em rule is only used for marking dialogues (in a novel, for example).
As a side note, I've done some research indicating that em rules are used for strong interruptions: see this for instance. I suppose the provided examples fulfill this condition (if not, I'd be grateful to know why).
Thank you in advance!