The control the mind exerts on the heart is complete and decisive.
For the situations in which control is lost do not occur on their own.
A person loses control when he allows himself to cross a certain threshold.
If he allows an emotion or relationship to develop beyond all manageable proportions he is liable to reach a point of no return.
He then becomes subject to emotions and desires that he can no longer stop.
"The mind controls the heart" does not imply that the mind can stop any activity of the heart or develop it by force of will alone.
Rather, it expresses the axiom that a person in a state of awareness and consciousness can supervise himself and refrain from certain actions and certain thoughts by not completing every thought that arises within him.
This ability not to think through every thought but rather to disrupt it at a certain point is an innate advantage of the mind over the heart, of conscious will over unconscious drives.
–Rabbi Adin Steinsaltz
From Opening the Tanya, p. 296 by Rabbi Adin Steinsaltz