MTB IS that much different from road cycling. MTB climbs will get the air out of your lungs much faster than a road climb. Learning to maintain momentum is also much more crucial in MTB than in roadie. There are many short climbs on the trails that can only be conquered by keeping momentum, no way to power through them. And obviously a lot more handling skill is required. Keep at it. You'll see that MTB will help your road biking, and road biking will help your MTB (increasing your cardio).