I know washing with hot water can cause it to be a bit of a mess but I bet most of it is just genetics and how it is. Try only using cool water when washing your beard and see if that makes a difference. A few guys with bigger beards on here, Weirdywithabeardy and Hargar recently posted pics after they had used a straightener and to be honest I wasn't a fan of the look (no offence guys). I think it's pretty much impossible to correct any natural curls or waves in your beard. Mine has tons of curls and I really don't think that can be changed.

Also, welcome to the board!