So that's the exact same story for me! I started off with a road hybrid and discovered that I liked riding in the woods much more than I did on concrete. There is a lot of skill involved in mountain biking and it's fun to learn each technique: lifts, rolls, drops, jumps, rock gardens, climbing, and turning (I didn't even know there was a specific technique to turning on dirt but it's really important).

Expect to spend about three grand for a good full squish mountain bike. I have a Rocky Mountain and I would highly recommend the brand. They are one of the best values in terms of features, weight, and quality. There are even some really good direct-to-consumer brands now like YT. For mountain biking you definitely want disc brakes, a dropper post, and around 120mm-130mm of front travel with a total weight less then 28lbs. Global Mountain biking network on YouTube is a great resource as well as local riding clubs and bike shops. Good luck!