Build it and they will come
Not true. You can build the greatest thing ever. If nobody knows about it, nobody will care or use it. Building it is half the battle. The other half is getting the word out, getting people/clients to use it, to love it, to need it. It gets more complicated even. When you're building, you may not know what your future customers will want or need. You may not even know exactly who those customers will be. So, how can you make a product that they will appreciate? First find the customers, and talk to them. Have a clear strategy to get new customers onboard. Do some market testing. Then build it. But build it incrementally, constantly going back to market research, and making sure it is a good fit with customers, that you still even have customers.
You invested so much time/money/effort into something. Just a bit more, and it will be worth it. Not so simple. Sometimes indeed adding a bit more time/money/effort will complete the project/product/business. However, the thing to remember is - when making the decision to continue or quit is - you must not consider anything that happened in the past. The past is gone, and no longer exists. Is the project worth pursuing now at this time, with your current knowledge? If it's just not working out, if you've learned that it is a bad idea that nobody is interested in, or if you realize now that it will take a lot more effort, then stop and consider the new reality. Pretend you're starting the project now, and have gotten whatever you have already built for free. Consider how much more time/money/effort it will take to make it a success. Is it worth it?