We clearly are talking at cross puposes here. Maybe it would be helpful to define terms. I simply googled these terms and agree with the following definitions:
Reason = the power of the mind to think, understand, and form judgments by a process of logic.
**Knowledge **= facts, information, and skills acquired by a person through experience or education; the theoretical or practical understanding of a subject.
**Doubt **= (as posted previously) Uncertainty, indecision, mistrust.
**Undermine **= damage or weaken (someone or something), especially gradually or insidiously
**Determinism **= the doctrine that all events, including human action, are ultimately determined by causes external to the will. In Catholic theology determinism is contrary to the doctrine of free will.
Free will = (from the Catechism of the Catholic Church) Para. 1731 “Freedom is the power, rooted in reason and will, to act or not to act, to do this or that, and so to perform deliberate actions on one’s own responsibility. By free will one shapes one’s own life. Human freedom is a force for growth and maturity in truth and goodness; it attains its perfection when directed toward God, our beatitude.”
If you agree with these terms we can look at your 6 questions.
First, how do we construct reason?
By taking in information (either through our own experiences or passed on from others) analyzing the information and making judgements.
Second, how could you be determined when you have doubt/are undermined (when you are trying to make a decision)?
In my earlier answer I thought you were using the other definition of determined (that is having made a firm decision on my own and being resolved not to change it) If instead you mean pre-determined by some outside force, I don’t believe in the doctrine of determinism. So it’s quite possible to have doubts about a decision. I don’t understand how you mean “undermined” here. Undermined by what or whom?
Third, where does reason come from when you are undermined?
Again, undermined by what or whom?
Forth, how could you have doubt if you are determined?
I don’t believe I am determined (by an outside force).
Fifth, what is doubt?
Already defined.
**Sixth, what is your definition of free will? I can define it as the ability that appears when neither doubt nor reason can win a battle. **
I go along with the CCC definition of free will. As such, it is quite possible to be doubtful or reasonable and still exercise free will in my final decision. In fact, by my definitions it is quite possible to be both reasonable and doubtful since they are not contradictory terms. I can use information to make judgements by reason but if I don’t have enough information I can have doubts. You seem to be using those terms differently than I would.