1. Mike thought Sue would eventually marry him. Now that she has married Tony, he’s eating his heart out
2. Kevin tried to take the job that was rightfully mine by telling my boss that I had stolen money from the company. When I got the promotion anyway, all I could say to him was, “ Eat your heart out ”

Compare to: cry over spilled milk. Whereas cry over spilled milk is to grieve over some event that has happened and cannot be changed, eat one’s heart out is to grieve over an emotional situation that cannot be changed. The expression is also used in the command form by someone who has no sympathy for the grieving person (as in opt.2).