One idiom that might suit this situation is "It's like water off a duck's back." Here is the entry for that expression in Christine Ammer, The American Heritage Dictionary of Idioms, second edition (2013):
like water off a duck's back Readily and without apparent effect. For example, The scathing reviews rolled off him like water off a duck's back. This expression alludes to the fact that duck feathers shed water. {Early 1800s}
Another water-related idiom that might suit the occasion is "It's water under the bridge." Here is Ammer on that idiom:
water over the dam Also, water under the bridge Something that is obver and done with, especially an unfortunate occurrence. For example, Last year's problem with delivery are water over the dam, or Never mind that old quarrel; that's water under the bridge. These metaphoric phrases allude to water that has flowed over a spillway or under a bridge and thus us gone forever. The first term was recorded in 1979; the variant dates from the late 1800s.
And a third potentially relevant idiom (suggested in a now-deleted answer to this question by Kaycee) is "It went in one ear and out the other." Again, from Ammer:
in one ear and out the other Quickly forgotten, as in Their advice to her just went in one ear and out the other. This expression, a proverb in John Heywood's 1546 collection, conjures up a graphic image of sound traveling through one's head. {Late 1300s}
English also has many idiomatic ways of saying that a person is not going to let some minor unpleasantness or annoyance become a long-term problem or concern: "I won't let it weigh [or prey] om my mind"; "I won't stew about it"; "It's no big deal"; "It's already in the rear-view mirror"; "I've already moved on from that"; "Life is too short [to hold grudges over something so minor]"; "I don't sweat the small [or petty] stuff"; etc.
