I presently ride a Dahon folding bike.  Today I tried to ride a mountain bike, and it felt unstable.  I felt unsure of myself so I only rode for a moment because I felt uncomfortable.

It was very heavy to ride, and required a lot of strength vs my folding bike.

It was difficult to start off, and felt shaky.

Will I get used to it?   What can I do to get comfortable on this bike?