Give 110% to every important thing you do.
The extra 10% is usually the clincher. After giving your full dedication and going the extra mile, you need to move on.
If things still don’t work out, deeply understand the reason so you learn from the experience and apply the learning going forward. Usually, things do not work out because the trust and rapport are not strong or you have not understood and solved the real problem.
Sometimes, it could be that you have not articulated the solution immaculately or been able to understand and surmount the objections. At other times, it does not work out because a third alternative has not been fully explored. The third alternative is neither yours or mine, but a better alternative!
However, once you have done all of the above then you can move forward in peace. Once you have done your best, leave the rest!