Just thinking "I can do this!" doesn't mean it is so. Not every idea is a practical one, and even if it is, it's not always appropriate to pursue for various reasons.
Something that can help is introspection: write down your ideas on a piece of loose leaf paper, one idea per sheet. Put them in one of those binders with the plastic pockets in, keeping the ideas separate. As the reasons to not work on it come to you, write them down on another sheet of paper, and put that in the same pocket. Do the same with the reasons for your idea.
Be completely honest and blunt while writing. Once you've done that, put the folder away and don't think about it for a few days or weeks. When you've forgotten most of the specifics, with no emotional investment in either list of your reasons, pull out the folder re-read the description of the idea, and the reasons for and against it.
Evaluate each one, always being honest with yourself, and decide whether each is a technical, rational, personal, or emotional reason, and proceed from that analysis. (Technical, "My skills aren't there yet." Rational: "I need funding in this field." Personal: "I have a kid and can't commit the necessary time." Emotional: "I'm no good and can't do this.")
Understanding why you thought what might be a good place to start.