I find it's helpful to write down the idea and let it sit for a while. Then I make a pros/cons table on a different day and try to compare the ideas on a scale

I find it's helpful to write down the idea and let it sit for a while. Then I make a pros/cons table on a different day and try to compare the ideas on a scale of 1-10 on various metrics that seem important. Often the decision becomes clear at this point. If it's still not clear, toss a coin and monitor your reaction. If it's negative, take the other decision. Stay nimble down the road and reconsider if you made the right decision, but with a bias to sticking it out with what you chose. It's better to make the wrong choice and realize it soonish than to make no choice at all. I have a problem getting distracted by new shiny ideas, so I don't allow myself any real pivots until I at least get past the MVP state. I tell myself to finish and launch it, even if nobody uses it, just to prove I can finish stuff.