Some people who shoot longbows with straight grips like to hold them firmly. I've never heard anyone who shoots a recurve or a longbow with a contoured grip who believes you should grip it firmly.
If you hold the bow so the pressure point is to the thumb side of your lifeline, when you draw the bow, all the force should be back against the pad below your thumb, and you shouldn't have to grip the bow at all, other than enough to keep it from flying out of your hand when you shoot it. The reverse should be avoided too; don't hold your hand with your fingers open (unless you use a bowsling) so they reflexively grab the bow when you shoot it.
Perhaps your bow arm would be more stable if you consciously push toward the target until your shot is released. You can tell if everything is working properly if your bow recoils toward the target on release, and not to one side or the other.