Rejoice in the Lord always; again I will say, Rejoice. Let your reasonableness be known to everyone. The Lord is at hand; do not be anxious about anything, but in everything by prayer and supplication with thanksgiving let your requests be made known to God. And the peace of God, which surpasses all understanding, will guard your hearts and your minds in Christ Jesus. Finally, brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things. What you have learned and received and heard and seen in me—practice these things, and the God of peace will be with you.[…] I have learned in whatever situation I am to be content. I know how to be brought low, and I know how to abound. In any and every circumstance, I have learned the secret of facing plenty and hunger, abundance and need. I can do all things through him who strengthens me.
Phil 4:4-9 & 11b-13 (ESV)
Being at peace with a decision is not the same as having the peace that passes all understanding. Making a decision that falls in line with your own desires is likely to produce a peace about the decision, but having a peace that passes all understanding is being at peace with the God of the universe through Jesus the Christ even though we deserve no peace from Him because we have committed treason against His infinite attributes (in other words, we have sinned against Him). If you have THAT peace, you can be at peace no matter what situation you are in regardless of whether it lines up with your desires or not.
“It is a by-product of the presence of the ‘God of peace’ that guards our hearts and minds in Christ Jesus.” The full reality of that peace comes to bear as we obey the command of verse 8 above: “Finally, brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things.”–in other words, saturate your thoughts in Christ & the Gospel. (Thank you Pastor Gary for that helpful insight!)