Step1
Self Judge- Ask yourself, what went wrong in your perfect relationship?
Step2
Evaluate Your Lovers Needs & Wants- You seem to be doing everything perfectly & are all praises for your self on the way you handle the relationship, but is that enough or is that all he wants, are you lagging behind in an area which can prove vital. There is a difference between need & want. See if he needs you or just wants you.
Step3
Spend Time Alone- It is needed to reaccess & reevaluate what all you did and how perfectly, if that's the best you can do-Good; if you are leaving a stone unturned, then maybe that stone is all that needs to be uncovered.
Step4
Now when you have reevaluated the minutest details then you can reach two conclusions
1) There was something lacking from your part.
2) There was something lacking from his part.
The first one is easy, if you have figured out the thing that was lacking from your side then you can work on it and set things right.
The second thing is a bit complicated as you will have to talk your way through to your lover of your expectations and what you want from him. In love most of the things are just not said but felt, but sometimes saying really helps.
Step5
Forgive- If there is a past grudge that you are carrying then that needs to go. Reflect back on the times that makes you happy & all the fun moments you have had together. The most destructive factor in a relationship is to keep reciting the bad things done by the partner; and the partner gets so done with taking all the snagging that he just says fine i quit. So always forgive and move on.
Step6
Having said and done all this. If the relationship was perfect and your lover still left you to be with another person then the best that can be done is to give him time to realize your worth. It often happens that you cuddle up your partner so much that he just starts taking things for granted and the relationship just suffocates.
Step7
Surprise- We all love surprises, don't we. Probably what ever you do is so mundane and banal that your lover already knows and can predict what is coming next; or he at-least has a clue. So he is probably not interested. Give him the unexpected, let his mind just wander about thinking ohh wow this is so cool. This will add some spice and things will get exciting. If you are giving him the freshness and excitement that he looks for in other people; then he doesn't have to go anywhere.
Step8
Skip you daily habits of which he is so accustomed of. If you call him daily at work then skip doing it for a day or two and you can see what happens. He will get so confused, & so uncomfortable that in the end he will end up calling you.
Step9
You are worth It- Show him that you can live without him/her and that your world doesnt revolve around him(atleast pretend). You have been constantly cajoling him & the result is that he has turned into a bad boy. Momma needs to teach the boy a lesson. Make new friends; go out watch movies, shopping or just have fun. That will help you get your mind off him and this will work as an antidote and your lover will nuts seeing that you have learnt to live without him. He will take some time to realise this and then when he will come back to you he will be all yours.
Get your love back
Novice 
