I've been making promises to my manager who is counting on me to help develop a big project but she doesn't know I've been looking for another job.
Why do you wanna have a different job and actively lying to your boss?
I'm smarter than my manager and I'm looking for new challenges
It's OK if you do not feel happy or you do not feel challenged anymore with their current job and want to find a different one. But I don't think that actively being deceptive with your boss is a good way to treat them especially if they have treated you well in the past. You should tell them you are considering leaving.
Yeah I'm gonna have a meeting with my boss.
Honesty is the best policy and who knows maybe they will be able to offer you a more challenging job at your company.