I hate fitted sheets. Hate them so much. They are always difficult to put on the bed as they always tend to pop off in the middle of stretching it over the mattress. They also always end up loosening up over time. My solution: industrial Velcro.
Basically, I glued some Velcro to the underside of my mattress and then sewed some Velcro bands onto my sheet. Now when I put the sheet on my bed I can secure it tightly using the Velcro straps. I’m loving this setup as the sheet never gets loose over time.