Lots of people have problems with gripping cards, and even experienced cardmen can lose their grip once in a while. So don't be too hard on yourself, card handling is something that has many aspects that take time to master.
If your hands are too dry, any moisturizer you like will do. Glycerin and rosewater is something that I've personally used and that I've heard a lot of the older magicians in Toronto recommend, and Dai Vernon, Ed Marlo, and Frank Garcia all famously used something called "Chamberlain's Golden Touch Lotion" which apparently is hard to find in stores but of course can be found online.
There are also little tricks to keeping your hands moist and ready for specific sleights. Off the top of my head, one that I use is to quickly run my thumb down the side of my nose as if to subtly scratch it, and I do this whenever I need friction on the thumb (for example in second deals).
One more thing: don't get too caught up in just trying to master the overhand shuffle. Learn it, practice it, but don't just focus on it because as you learn more things you'll end up practicing it along the way automatically. It's one of those moves you always kinda use and get better at.
Good luck and keep practicing!