It's not illegal to break into your own mailbox as long as your mailbox is in your yard.
Now if you have a mailbox in a post office building and then break into it to get your mail then you can be prosecuted because the mailbox actually belongs to the post office and you would be destroying and breaking into federal property.
So yes it would be illegal for you to break into your own mailbox at a post office inside a postal building.
But if you have a mailbox at home that you get your mail in and you have a lock on it then you can legally break into your own mailbox to get your mail.
If you lost your key or combination to your mailbox in a post office then speak with someone in the post office who can help get your mail or get you a new key or let you know the combination.