If the shoe fits, wear it
If a description applies to you, then accept it.
This expression is often used when something derogatory is said about a person who then complains to a third party. The third party, if they agree with the original negative comment, might suggest "If the shoe fits, then wear it". An example of that might be:
Jack: Just because I've missed two or three sessions, my fitness trainer says I lack motivation.
Jill: Well, if the shoe fits, wear it.
'If the shoe fits, wear it' is often shortened to 'If the shoe fits...', leaving the listener to fill in the blank. The expression is the American version of the earlier British phrase 'If the cap fits, wear it', which is also still in general use. Daniel Defoe used the earlier phrase in the satirical poem The Dyet of Poland. Defoe had the work printed in London in 1705 but, as it was a rather vehement critique of the English parliament, Defoe used the flimsy pretence that it had been printed in Dantzig and was the work of 'Angliopoloski of Lithuania'. Defoe's point in the poem was that readers are responsible for their own opinions; he (or rather Angliopoloski) may have written the poem but that any conclusions drawn from it were owned by the reader, not him:
Gentlemen, and if the Cap fits any Body let 'em wear it.
'If the cap fits' is itself a version of a yet earlier phrase 'if the cloak sitteth fit', that is, 'if the cloak fits well'. This expression dates from the 16th century and was used in print by Richard Hooker in Of the Lawes of Ecclesiasticall Politie, 1593:
Which cloake sitteth no lesse fit on the backe of their cause, then of the Anabaptists.
The 'cloak' version of the phrase does suggest that the later 'cap' was a variant of 'cape'.
As to 'if the shoe fits', that began being used in the late 18th century. The earliest example that I have found in print is from the US newspaper the New York Gazette & Weekly Mercury, May 1773:
Why should Mr. Vanderbeck apply a general comparison to himself? Let those whom the shoe fits wear it.
The change from cap to shoe may well have been influenced by the Cinderella story, which has a snug-fitting slipper as the primary plot device. Versions of the tale that include the 'lost slipper' scenario were well known in the USA and Europe by 1773. In 1634, Giambattista Basile, published Il Pentamerone, a popular collection of Italian folk tales. One of the stories, Cenerentola, is the basis of the Cinderella story as we now know it, complete with wicked stepmother, ugly sisters and a missing slipper.
Many expressions, for example, 'toe the line', 'get off your high horse' etc., were first used literally and their metaphorical meaning came later. 'If the shoe fits' is a rarity in that it has gone the other way - having been used for centuries in a figurative sense, its most common usage now is in shoe shop advertising slogans.
Since writing the above piece I've had many reminders from US readers about the shaggy dog story concerning the mythical Foo bird, the droppings of which were supposed to be lucky and shouldn't be washed off. There are as many variants of this old joke as there are tellers of it; the only common element being the punch line "If the Foo shits, wear it".