Commitment is emotional investment
"A real decision is a ... personal commitment to a choice or option, or a group of them."
Theodore Isaac Rubin, Overcoming Indecisiveness: The Eight Stages of Effective Descision-Making, p.10
Commitments, things you have already committed to, also known as duties and responsiblities, make it easier to come to decisions which will support those pre-existing commitments. Contending or competing commitments make it harder, which makes a strong case against taking on too many commitments.
"In very few instances is one decision actually better than another."
Rubin, Overcoming Indecisiveness: The Eight Stages of Effective Descision-Making, p.70
Most actually "bad" choices are discarded early in the process, before you start consciously evaluating your options. Commitment to seeing your decision through is what makes it work. If you get actual evidence that you have made a bad choice or that the situation changed enough to make a different option substantially better than your current choice, THEN you can (and should) change it.
Maintaining Momentum: Motivation & Enthusiasm
You need to make a vigorous start. Enthusiasm is a motivation to get started, one of the best in fact, but without the determination to finish, won't get you very far. Your commitment to the project is what will keep you going as your enthusiasm flags. If your project is substantial, taking any significant time to complete, your enthusiasm will flag and return several times during its course - provided you can keep going between times.
Commit through Action: Do It!
Commitment can keep you going, but the best way to stay commited is to just keep going. Do something on your project, no matter how small, to help get you moving when you don't quite feel like it right now. If you let that feeling keep you from working, you will usually find yourself working less and less as time goes on. That is one reason most people cannot successfully work for themselves or study without schools. They just don't have the self-discipline to keep at things even when they don't feel like it.
Decisiveness, Perseverence, and Fortitude are slightly different aspects of the resolution to accomplish goals you have planned, except when you consciously change them for a good reason.
Environment can be hindering, so you need to work through periods when your environment is not helping you. And you need to change the environment to reduce hindering effects, because it will wear you down and reduce your effectiveness.
Commitment to Ends
The more distant and general ends (such as your Values) you commit to will allow you to be more flexible in your responses to changing circumstances. And therefore more likely to actually benefit from your commitment. Working toward a more distant goal can help reduce stress by reducing or softening deadlines. A sense of play about the goal will also help reduce stress and maintain commitment.
Avoid commitment to means. Ends are primary and you may need to change means several times to get there. Paul Graham funds startups and has written many essays on how to operate in one - one of his most consistent themes is the need to be open to changing direction - that most successful startups had to change their plans substantially at some point as they learned more about their potential customers and the product they were creating. (I linked to his homepage rather than any particular essay, because so many bear on these ideas. Also, most of the essays are very readable and interesting even when they don't.)