decisive- someone who is decisive is good at making decisions quickly and with confidence However, a determined person may have a strong desire to do something, and will not let anyone stop him/her. BUT he/she may not necessarily be a confident person, nor be able to make quick decisions..

Determined people exhibit the Power of the Will. Decisive people exhibit discrimination in thought. .

I don't feel that either carries the meaning of "self-confident". I agree with the Longman definitions. Confidence in decision making does not necessarily imply self-confidence. A person with self-confidence may make quick decisions, or they may take time to ponder, gather data, ask questions, etc. Their decisiveness may come from experience or their approach to problem solving. They may also lack the character or ability to carry out their planned action. A determined person may be quick or slow at making a choice, but once committed, they work to see that the action is completed. The determined person may not be confident in any other area of their life but they have their mind set to complete something once they begin.