Imagine reaching toward something in front of you with one arm, stretching, stretching, stretching as far as you can go... while the other arm is holding on to something with equal force behind you. Clearly, there would be a serious limit to how far the reaching could extend, even if you were reaching toward something that was very desireable.
Opening to something new is like that. It involves reaching with one arm and letting go with the other. Holding on to what is behind you prevents the new from emerging. Therefore, no matter how much one desires a goal that the mind or heart may choose, the desire must be accompanied by a letting go in order that the new may appear.
Both mind and heart can be made ready to let go to the extent that each becomes rooted in a relationship with God. Then, one does not let go into emptiness or fear, even while the goal has not yet been reached. One lets go into love and trust. One lets go into God.