When exactly do we create new karma when every moment of our life is destined?

At birth, a person’s amount of suffering and enjoyment in life is fixed.

Just because your life is pre-destined, however, doesn’t preclude the fact that every action you do, creates a reaction.

Simply put, it can be said that the suffering and/or enjoyment you experience right now is the accumulative result of activities performed in the past going all the the way back to past life-times.

And the activities you perform right now will result in your future suffering and enjoyment. The fact that you, by your actions, create new karma all the time, does not rule out that you were destined to act in a certain way.

We may think that we act on our own choice, but the fact is that the only real free choice we have in life is whether we want to be controlled by nature, controlled by our karma, or whether we want to be controlled directly by God, controlled by Krishna.

Krishna says:

Everyone is forced to act helplessly according to the qualities he has acquired from the modes of material nature; therefore no one can refrain from doing something, not even for a moment. — Bg 3.5

One who restrains the senses of action but whose mind dwells on sense objects certainly deludes himself and is called a pretender. — Bg 3.6

On the other hand, if a sincere person tries to control the active senses by the mind and begins karma-yoga [in Krishna consciousness] without attachment, he is by far superior. — Bg 3.7

Perform your prescribed duty, for doing so is better than not working. One cannot even maintain one’s physical body without work. — Bg 3.8

Work done as a sacrifice for Vishnu has to be performed; otherwise work causes bondage in this material world. Therefore, O son of Kunti, perform your prescribed duties for His satisfaction, and in that way you will always remain free from bondage. — Bg 3.9

Srila Prabhupada explains:

Since one has to work even for the simple maintenance of the body, the prescribed duties for a particular social position and quality are so made that that purpose can be fulfilled. Yajna means Lord Vishnu or sacrificial performances.

All sacrificial performances also are meant for the satisfaction of Lord Vishnu. The Vedas enjoin: yajno vai visnuh. In other words, the same purpose is served whether one performs prescribed yajnas or directly serves Lord Vishnu.

Krishna consciousness is therefore performance of yajna as it is prescribed in this verse. The varnasrama institution also aims at satisfying Lord Vishnu.

Varnasramacaravata purusena parah puman / visnur aradhyate (Visnu Purana 3.8.9).

Therefore one has to work for the satisfaction of Vishnu. Any other work done in this material world will be a cause of bondage, for both good and evil work have their reactions, and any reaction binds the performer.

Therefore, one has to work in Krishna consciousness to satisfy Krishna (or Vishnu); and while performing such activities one is in a liberated stage. This is the great art of doing work, and in the beginning this process requires very expert guidance.

One should therefore act very diligently, under the expert guidance of a devotee of Lord Krishna, or under the direct instruction of Lord Krishna Himself (under whom Arjuna had the opportunity to work).

Nothing should be performed for sense gratification, but everything should be done for the satisfaction of Krishna. This practice will not only save one from the reaction of work, but also gradually elevate one to transcendental loving service of the Lord, which alone can raise one to the kingdom of God.