Think about the indie game devs out there. We're often making a game we saw that we think needs a twist. It's the same with my first SaaS idea I'm starting. I was interacting with some common software and thought, 'Man the UX here is unpleasant.' Chatted with the guy who used it and he told me how much per month it is, for what it is, it costs way too much imo.
Just because there's an app out there, doesn't mean it's going to do things the way you'd design it. That could change it. If you find an app doing the same thing, maybe that means you tweak the idea.
I think you just need to ask yourself, do you still believe in the idea?
I'd also personally consider doing some market research when you can
I get an app idea.
I start building it because I find it useful.
I see other alternatives on the internet.
I get demotivated and stop working on it.
Never release it.
Repeat with other app ideas.
How can I overcome this crap?