daunting
Seeming very hard and making you feel nervous or unsure.
Daunting means intimidating or discouraging because something seems very difficult or overwhelming. When a task feels daunting, it makes you hesitate because you're not sure you can handle it.
A fifth-grader might find a 500-page book daunting at first, especially if they've only read shorter books before. The sheer size makes it seem impossible to finish. Learning to play chess can feel daunting when you first see all the different pieces and rules. Standing at the bottom of a tall climbing wall and looking up at the top can be daunting, even if you know people climb it successfully every day.
The key thing about daunting challenges is that they seem overwhelming, but that doesn't mean they actually are. Most daunting tasks become manageable once you break them into smaller steps. That 500-page book becomes just twenty pages a day for a month. The climbing wall becomes one handhold at a time.
When something is daunting, you feel a mix of nervousness and doubt. But daunting challenges often turn out to be the most rewarding ones to tackle. Something daunting hasn't stopped you yet, it just looks like it might.