Do your best, leave the rest! What does that entail?
Give 110% to every important thing you do. The extra 10% is most often the clincher. After giving it your full dedication and going the extra mile if things still don’t work out, try and deeply pinpoint the reasons why so you learn from the experience and apply the learning going forward.
Usually, things do not work out because the trust and rapport is not strong or you have not understood and solved the real problem. Sometimes it could be that you have not immaculately articulated a common solution or been able to understand and surmount objections.
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 suited for both!
So in the end, if you have given it 110%; learned from the experience, and explored a balanced third alternative, you can be at peace with moving forward.