Post launch: when should we pivot or call it quits if our startup isn't getting many users?

It always happens when I launch new apps. A month or two into marketing it, there is very little growth or paid customers. Which leads to me losing interest or getting pessimistic about the app I've spent months building.

How would you tell post-launch that a startup isn't going to work and it's time to either move on to something else or try to pivot the business into another concept?

What's your app? Maybe the problem isn't your app. Maybe it's the way you are marketing. It's a big world, no one will know you exist if they never see you. Think of ways to be discovered. If you move to another concept will you hit this wall again? How will the next app be any different if you can't get it in front of them. Go find your people!

Negative thoughts run through my head all the time. How i react to those thoughts makes the difference.

I agree with Andrea. Perhaps first identify the market. Create an app with minimum features and allow users to direct where the app ends up. Also, read "The Lean Startup" by Eric Ries for insightful advice on this specific problem you facing.

First: do the toothpaste test - will I use this app at least once or twice a day? If not, then it might be the problem.

Second: You need to get critical mass to become a succes. Hype, pr, ads - whatever it takes.

Last: Use your data. Does your current users really use your app? If not, then your product might simply just not ve good enough.