How to get out of the shell, to stop thinking "do I really can?" and start making things I've ever wanted, like expand my business?

This is a typical question for those who have yet to venture into operating a business, assuming the business you mentioned is profitable and without shrinking demand you should already feel comfortable to operate it and expand it because growing a business, except for pivoting and or seeking investments, is really nothing more than hiring more sales people, putting in more hours, being friendlier to your customers.

Obviously each of these implies its own "to-do" lists but you can start tackling it with follow up calls, end of work surveys, referral discounts, offering limited time only options, match competitor prices, etc...

I hate to say this, but many questions are better asked over the phone and better explained and results. Give me or any applicable person here a call and let's talk about your business in particular and what approach can help you change your mindset by applying easy to track changes into your business :)

I've seen that syndrome over a hundred times while mentoring and teaching courses and everything can sum up in a single term: Motivation.

The low level of motivation is the reason for you to keep asking that question without taking action.

Lack of motivation is caused by different factors, and could be tackled with different actions. The easiest way to fight that is by leaving yourself no choice.

"Just do it", as a popular brand says in their motto. If you work full-time, quit. All of your income should come from your business venture, so you better get that going ASAP.

If everything is too vague, break it into groups and subtasks. Define everything in details and set milestones. Deadlines are always a good motivator. Announce them publicly to everyone - if you count on your name, you better not fail that.

If you are afraid of the competitors, find your unique selling point. Beat them with a killer feature and sell it at your best.

Whenever my mentees or students have doubts, I outline them the alternative. A boring 9-6 job, a mean manager, tedious tasks, lack of progress. Planning in iterations with pressuring deadlines leaves you no time to think of "can I" since you're too busy to execute the plan and progress further.