Temperament is determined by your unique neurological characteristics and unlike personality, it cannot be changed. It’s like handedness; a set of inclinations each of us are born with, a predisposition to certain attitudes and actions.
If you were born left handed, you will always have a preference for using your left hand, even though you can train yourself to use your right hand more than you otherwise would. If you force yourself (or someone forces you) to become right-handed, it won’t work and you will only become frustrated. Temperament is the same way. If you have a natural preference for spontaneity and flexibility over being decisive and organized, you will always be that way. You can train yourself to be somewhat more organized, but if you try to completely alter your temperament you will only become frustrated and depressed. Temperament differs from personality, which is a combination of your temperament and life experiences, although the two terms are often used interchangeably.