New developers underprice their first three projects almost without exception. It's not a confidence problem — it's a scoping problem. If you can't break a project into tasks, you can't estimate it, and if you can't estimate it, you default to guessing low.
Before quoting anything, write a feature list with the client and mark each item as 'must have' or 'nice to have.' This single step prevents the single biggest cause of freelance disputes: scope creep that neither side agreed was in scope.
Quote in ranges, not fixed numbers, for your first few projects — a low-to-high band based on your task list gives you room for the inevitable unknowns without underselling yourself.
Finally, always add 20% to your time estimate before converting it to a price. Every developer underestimates; padding the number instead of the promise keeps your delivery dates honest.