If you find a product similar to what you're building, will you continue building it?

I think you need to figure out what differentiates your product from theirs. Whether you're targeting a different market or if your product will solve a different problem or is your solution is somehow better.

Yes, by finding a deeper niche in that space. The best example would be Carrd vs Wordpress/Squarespace. 'One page sites' can be built using Wordpress/Squarespace but instead of using them Indie guys and small startups started using Carrd to quickly make a landing page to test their MVP, hence a small niche was carved out by Carrd

I personally feel that it's the execution of the idea which matters the most. Nobody can push it like YOU can. So you should analyse, think about what differentiates your product and definitely go forward with it if you are certain that NOBODY can do it like you can! All the best! I used to feel the same about my website, www.nowandme.com, but I took the leap and went forward with it!

Yes, maybe add some other unique feature or reduce cost to differentiate it.

I would first try to find out the weaknesses of the existing product and then check if your idea offers an benefit / USP for the target group. In my opinion it is best to start with surveys and user tests before building the platform.