Do you want to find your new congregation before you leave this one? Or are you committed to leaving this one, regardless of what you find?
If it’s the former, then I recommend letting your pastor know that you are taking some personal time to think, reflect and recharge your faith and that you will be less available. Start going to other churches to see if their approach to faith resonates with you. Keep your old church at arms length. When you are ready to leave, just send the pastor a note saying how much you appreciate his commitment to his congregation but circumstance and reflection have led you to another flock and you wish him all the best.
If it’s the latter then just switch the steps. Write to your pastor letting him know that you’ll be leaving for personal reasons and that you wish him all the best. If you think he or anyone would hound you to return, you can add that you don’t wish to be contacted and your decision is final. Then you can take your time visiting new churches.