Front hydraulic disk brake is too powerful on MTB — solutions?
It's hard to say exactly what the cause is, but I see two main possibilities: 


Mechanical causes


Brake pads.   If the pads are very close to the rotor when at rest, and have minimal motion to close on the rotor then that may be enough to wedge them.  Try inspecting the brake pads and if worn, replace them.  Brake pads are consumable.  Thin brake pad material will reduce your "modulation" zone, same as hard brake pads on rims do.   Could consider swapping your pads front to rear caliper temporarily as a test.
Pistons. If your pistons are a little gunked up at the sides, they may not be retracting fully.  This may be limiting the distance that the pads have to travel to be hard on the wheel.
Rotor wear and damage.  Spin the front wheel off the ground and look closely at the rotor&mdash;it should not wobble or divert side-to-side, and the friction surface should not have burrs or obvious wear.  
Fluids. I doubt these are the cause of your issue.  Low fluid, or air bubbles will exhibit as spongy-feeling levers and you don't have that.  (edit) though @t_bacon points out that if the system is over-filled it will be slightly pre-pressurised, which will be pushing the brake pistons out a little all the time.     It's definitely worth checking if the fluid is over-filled, by opening the bleed screw and see if fluid squirts out at all.

Technique


When braking, your lever should have a range of motion before the brakes are wedged hard-on.  How tight you squeeze your fingers controls the level of braking effect.   There's a great answer at What does &quot;modulate&quot; mean when referring to brakes? Just visualise your flatbar lever instead of road brake levers, the concept is identical.
Body position for fast braking should be "arms straight and arse back"   If you try and stand up on the pedals while hard braking your center of mass is higher and theres a moment-of-rotation which robs your rear wheel of traction and imparts a forward force on the rider.   



One suggestion would be to swap bikes with a friend and try some carpark laps on another bike equipped with hydraulic brakes, and to get that other rider to try your bike brakes.  Decide if its a problem endemic to your bike vs technique, and go from there.