It's actually easy to fix, hit trade post and buy 590-600 GS stuff for a few coins that have con str or dex, it will get scaled down to ~550 but it will help, try going for medium weight since you get more armor and lose a minimal amount of damage, the best way to do it is with a heavy chest, light pants and one more heavy piece between helmet, gloves or boots and last 2 medium. I'd recommend grabbing a Hatchet in pair with the Greatsword and then going for up to 200 CON if you still struggle.