Find another. In FULL. It's not the only headboard on the planet. You are shoving a 54 inch mattress up against a 60 inch wide thing. It's probably junk and WIDER than 60 inches. Which for queen it should not be. Send it back.

Better yet? Get a full size BED, not just the headboard. A guaranteed fit. Or trade up the mattress to queen, where the options will be far more plentiful

Thanks for the advice! Problem is, I love the headboard and cannot find one like it for my new adjustable full size mattress. Your advice is right on. I will probably look for a full size bed. Lina

Would have been helpful if you had posted a picture of the problem. Are you sure you can't simply flip the brackets from the frame and get it to fit?

Here are adapter plates.

https://www.thesleepshop.com/store/pc/Mod-Adapt-Headboard-Plates-34p137.htm

Thanks for the info. However these brackets wont work for this problem.

Will a bed skirt cover up the brackets? Are you sure you installed it correctly? I have a Queen headboard on a full bed and yes the headboard extends past the width of the bed but the brackets are attached to the metal bed frame and so there is no way for them to show. The additional width on the headboard is balanced out easily once the bed is made with down filled duvet and pillows.