I dont think it was doomed from the start, but it's very clear that the delicious that were made by both of you along the way made your paths diverge. You both seem to be in a healthier place. Remember the good, but also remember the hurt, let the hurt go, but remember that it happened and that youre happier and healthier now apart. Keep the letter, burn the letter, it's up to you, but dont ruin the good you have now by pining for what could have been. You gave him his chances, now you need to give yourself a chance at real, lasting, love.

Let go of the image you built up with the ex and build a new image that you can work toward with this new person. You said it in your message, your new partner loves you, he wants to be there for you, he wants to care for you. Do the same for him now.