There's a reason why its hard... because you can't learn to be ambidextrous. You're either born a lefty, a righty, or ambidextrous. You can't "learn" to become a righty if you're a lefty and vice versa. The same applies to becoming ambidextrous.
Actually, that is not true.. it is the other way around. True ambidexterity, that naturally occurs, is rare. Only 1 in 100 ambidextrous people are naturally ambidextrous, the other 99 have learned it. I have learned it mostly by playing the drums for many many years (it's not just hands, feet/legs count too), and learned writing with my left hand during an extended period of recovering from a complicated fracture in my right arm.
But you can learn it, and the majority have. Only the odd one out has not and is born with it.