"Didn't" or "Wouldn't"?

Question

When I was in high school, a mosquito bite used to be itchy for many weeks. It didn't subside for a month or more.

Is the word didn't proper here or should I use wouldn't? Wouldn't sounds better to my ear but I do not know why. The itch used to last longer. That was a fact then; not any more.

EDIT: about my checked answer

The difference appears to be pretty subtle. One answer prefers simple last, the others think both are OK, but explained the context around it (different word usages). I think all three answers are valuable and worth taking a look. I up-voted all three, but decide to check one that provides a simple rule to follow. Thank you all.

Answer 1

«Didn't» is more appropriate for this case. But it's not so easy.

«Wouldn't» is for Future in the Past or Conditionals sentences and it's about ability, not the fact (the second and the third, like if I could ... I would or if I were you ... I would ...).

Here you're talking about past time using time period

for a month or more

so you have to use past continuous time like

They hadn't been subsiding for a month or more.

I'm not sure, but this version looks more grammatically right.

Answer 2

You can talk about things that happened in the past but don’t happen anymore in different ways using used to/would/past simple

The OP says

When I was in high school, a mosquito bite used to be itchy for many weeks. It didn't subside for a month or more.

Repeated actions in the past are normally described using used to or would.

So, ...it didn't use to subside for a month or more.

or

...it wouldn't subside for a month or more. In this case I'd probably link the two sentences with 'and' (...itchy for many weeks and wouldn't subside ...)

I might use the past simple too, but only if the repetitive nature of the action is acknowledged by adding 'usually' to the phrase

It didn't usually subside for a month or more.

• See more at: https://learnenglish.britishcouncil.org/en/grammar-reference/past-habit-used-towouldpast-simple#sthash.SwBhtfnd.dpuf

Answer 3

Would has come from the OE wolde, the past tense and past subjunctive of willan: wish, want or will.

So, although wouldn't and didn't are essentially interchangeable now, I think "wouldn't" in the context above can still imply some sense of "not wanting" to subside; some resistance to stopping.

We are more likely, I think, to use "wouldn't" than "didn't" in a sentence like:

"Whatever I tried, the itch wouldn't go away". It persisted; it did not "want" to leave.