You have to learn to move on with your life, he has moved on with his and you too will find someone else who will be that special someone for you.
I know it's hard, we have all been there and experienced losing someone we love and having to deal with seeing them again moving on with their life.
You should treat your relationship with that person as experience and good experience for future relationships. We all look back from time to time and wonder what if....etc, but standing still and dwelling on the past ends up being a waste of energy that you could be putting in to yourself or towards another relationship.
Hold your chin up and be proud that your doing something positive about your weight and carry that positive energy with you, that's what will get you the result you want for yourself and there will be a fair chance someone special will see that positive energy and be attracted to you. And here is a hug to get you started
