It's a special form of "to be" used in counterfactual clauses. That is when what you are saying isn't true: my heart is not a ball.
Here is a reference to that use:
Wikipedia "use of the past subjunctive"
"The main use of the past subjunctive form, were, which is also known as the irrealis is in counterfactual if clauses."

Another reference discusses the form extensively:
Thoughtco "past subjunctive"

In ordinary speech, in your example sentence, some people will use the form "was".

It's a special form of "to be" used in counterfactual clauses. That is when what you are saying isn't true: my heart is not a ball.
Here is a reference to that use:
Wikipedia "use of the past subjunctive"
"The main use of the past subjunctive form, were, which is also known as the irrealis is in counterfactual if clauses."

Another reference discusses the form extensively:
Thoughtco "past subjunctive"

In ordinary speech, in your example sentence, some people will use the form "was".