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Pete Wentz had no idea if he'd be a good father.

The 36-year-old Fall Out Boy frontman is father to six-year-old Bronx with ex-wife Ashlee Simpson and nine-month-old Saint with girlfriend Meagan Camper.

And in a touching letter written to his sons in honour of the upcoming Father's Day on June 21.

'You have both changed me and taught me so much in ways the version of me that existed before you would never be able to understand,' Pete wrote.

'Bronx: before you were born, I had literally never taken care of anything in my life. Whatever the word for the opposite of responsibility-that's how I would accurately describe my life then.

'During that first diaper change I ever did (which may have been a world record in terms of how long it took), you had that look in your eyes that said, 'Come on, you got this, you can do it'-even though I can admit now that, back then, I had no idea if I actually could.'

The letter, which Pete wrote for TIME magazine's ongoing Father's Day project, continued with the rocker saying he was slightly more prepared for the arrival of Saint.

However, Pete admits his life was once again turned upside down by the new arrival.

'Saint: when you came along, I was sure I was prepared,' he continued. 'I had my plan. I was the diaper-in-the-back-pocket dad. Spoiler alert: I was not prepared at all. You arrived like a little zen master, but even though I wasn't the rookie anymore, you let me know there will always be curveballs.'