On the road is fun but the dirt is "WooHoo!"
Good for you to pick up mountain biking!!
I have been doing it for two years now. I do not feel like a pro but I don't feel like a total newbie either. I ride plenty with my sweetie. He races - rides endurance MTB races. I have been fortunate to have a considerate but not condescending riding companion. Having him out there is a carrot for me to get the most out of myself. He gives me great tips and support but lets me "ride my ride".
Most frequently I ride alone and then over trails we have done together. I find if I do them at my own pace, stop to judge obstacles, and fly when I can, works well the next time we ride together. It definitely builds confidence. I also have some folks I ride with that aren't quite as skilled and that is an exercise in humility that is also good for the bike soul.
Ride with lots of different abilities and you will learn the most. Trust your own judgment, respect gravity, and have a blast!