Too scared for not being able to deliver?

For years I‘ve been learning about social Media Marketing and I‘m at a point where I would just like to start my own business and actually work with people/businesses together but I‘m kinda scared that I won’t be able to deliver because I know there’s never a guarantee that the marketing strategies for example will work.

Do you have any advice? Should I maybe put in more years helping Businesses for free to make enough experience or what’s the way here? Because my dream is really to make a good living out of something I actually love.

Sorry if my English isn’t that great but I hope you guys get my point. And if this is the wrong place to ask this please let me know