Unfortunately, fusibles aren’t as hardy to washing and wearing as they could be. Usually, these are used for a one-time wear project, or, more likely, used to keep items in place for applique.
I would recommend leaving the letter in place and then sewing around the edges with a zig-zag stitch on a machine to hold it in place. This is a pretty straightforward tutorial on machine applique. If you don’t have access to a machine, you can sew with small stitches by hand around the edges.
If you don’t want to sew it at all, you can use a stronger fabric glue to hold the piece down, though keep in mind that you may need to reglue at regular intervals if it continues to lift. I would also recommend washing by hand and drying by line if you go this route.
I hope that helps! Good luck :]