see also “Fate“
Destiny refers to the predetermined course of events. Indeed there are predetermined courses of events but these courses are determined by human beings themselves. The human being possesses a free will which he can use in the making of any decision. Once the decisions have been made however, the consequences can no longer be influenced by the maker of that decision. The consequences roll on and are automatically brought about by the decision.
It is very much like throwing a stone. One may indeed decide not to throw the stone, but once the stone is thrown it goes inexorably toward its target. It is then no more under the control of the thrower once it has left his hand. Destiny is very much the same thing. We have the free will to make decisions beforehand but once the decision has been made, the consequences of that decision are no longer under our control yet we still bear full responsibility for the decision.
Many of us, through many earth lives have used our free wills to make decisions, the consequences of which appear to us in subsequent earth lives when the starting points of such decisions would have been forgotten. When we are then hit by the consequences we say that it is destiny or fate and that we have no control over them.
The Light is able to see the consequences of each of our decisions and as such is able to predict and warn us to change our ways before the consequences hit us, but most of the time we do not heed these warnings. This is actually the basis of prophecies. Prophecies are given through particular persons to warn particular groups about an impending event. In some cases, it is to induce them to change their ways in time so as to avert some disastrous consequences. If such persons change their ways in time, then the disaster can be avoided, but if not, these consequences would have to hit these people with full force. So destiny is not always inevitable.