Fair enough, for a brushed motor, voltage control will give a fairly stable form of speed control.
You say from AC? The easiest way would be to use a Thyristor bridge to make a "controlled rectifier" and thus vary the DC-link feeding the motor (big DC-link cap).
Using a simple voltage loop to change the firing angle based on the present voltage error (Voltage demand - voltage feedback).
What circuits have you tried. It sounds like you have recified the AC and then used a H-brisge to chop it down.
Can you post what you have done.
Still a phase-control thyristor rectifier would be the better option