The Collaborative International Dictionary of English v.0.48:

Self-restrained \Self`-re*strained"\, a.
   Restrained by one's self or itself; restrained by one's own
   power or will.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "self-restrained":
   assured, balanced, collected, composed, confident, controlled,
   equanimous, equilibrious, firm-minded, forward, importunate,
   independent, levelheaded, poised, pushy, recollected,
   self-asserting, self-assertive, self-assured, self-confident,
   self-controlled, self-expressive, self-possessed, strong-minded,
   strong-willed, together, well-balanced