Your problem may be that the seat is too low.
Your seat needs to be adjusted in relation to your pedals, not whether or not you can touch the ground while seated. In fact, you shouldn't be able to touch the ground while seated, at least on a road bike. You want your knee to be just about in perfect line with your foot on the pedal with the cranks at the 9:00 and 3:00 positions. Adjust your seat accordingly, then use a sharpie to mark 1/8 or 1/4 increments, and raise and lower as you ride to find the sweet spot.
There are a lot of Hard Rock riders here, so hopefully some of the more taller folks can chime in on frame size. I'm just not that knowledgeable on MTB sizing.
Welcome to the herd! There is a lot of help and support here. You will enjoy the group.