I couldn't find this particular idiom "to pick at the scab" in any of the famous dictionaries online like Cambridge or Macmillan. I could only find it on Urban Dictionary
That makes me wonder if it's a correct idiom or not?
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Cambridge doesn't consider it as a set phrase, but it gives an example of it used in the literal sense under the entry of scab as noun:
a hard covering of dry blood that forms over a cut or sore:
- Don’t pick at your scab!
Urban simply states that it can be used figuratively, too. The phrase seems to be used more recently. Here is an example found in A Widow's Words: Letters to My Beloved, By Cathy Penman:
Similarly with a bereavement – don't 'pick at the scab' of the emotional wound, leave it alone to heal with time.
Verbs and nouns that collocate, like "have a shower" or "offer an apology", aren't considered idioms, so they don't get mentioned in dictionaries. The verb "pick" collocates with the noun "scab", so it's natural to use them together, but a dictionary wouldn't list the collocation.
To "pick a scab" means to pull a scab off. To "pick at a scab" means to play with a scab over time, maybe pulling bits off, perhaps without focusing on it much, the way people twirl their hair or doodle.