I have been there, and felt that doubt many times. It's an important emotion that you have to sort through before you take the leap.
Based on your description, I suspect that your hesitation is well-founded. Risk is not for everyone. Trust me when I tell you that the level of risk you feel now is minuscule compared to what you'll experience after you've invested your time and money into an idea.
It's not really about "growing a pair." It's about being mature enough to assess the business concept objectively. Everyone has "great ideas" for X or Y new product, app, service, etc. True entrepreneurs learn to discern the difference between a good idea and a viable business.
To do this, and be successful, you have to KNOW it will work. In my experience, and through the experience of my more accomplished peers, I can tell you that we all have ideas. But when that one special one comes along, it haunts you day and night. You just KNOW it.
I'd say, when you feel like that, pull the trigger. Until then, consider working for another startup to learn as much as you can about how to wade through it, and more important, how NOT to go about it. Also, consider taking your idea to an incubator mixer party and run it by a few people to help validate it.
When you feel that KNOWING, and you have the confidence that comes from having seen some of the ins and outs of startups, your balls will be plenty big enough to execute.
~Feel free to set up a call with me if you want to hear about how I got through this with my startup last year.