Hmmm. Tough situation Rahul. Thank you for sharing your pain. I am not in your place to tell you i understand, but I empathise. Heart breaks can be devastating and scar you for life. What is more important is that you need to know that it is not the end of the world. Fine, your life must have revolved around the lady in question. However, a deeper dignity exists in knowing that someone wants to be with you out of their own will. Some times it is a chance of destiny and luck, sometimes past karma (because i am a huge believer of karma). If you are not able to convince her to spend her life with you..then perhaps is it worth it? I am not asking you to hate her, because the good memories will be there, and there will always be a soft corner for her. Perhaps everyday at least once a day for a while you will ask this question as to why she didn't choose you? What did she see in the other person? why him and not me? I should have been in his place.
I feel you.
But you need to realize that, healing is a process. It will take time love. It is very important for you to not be hard on yourself. Accept what you are feeling. Don'r run from it. Once you realize what you are going through, you will come to the next step of dealing with it.
It will come slowly but healing will happen. It will make you stronger.
Secondly - please be careful to not blame yourself for her different choice of person. Perhaps the demons that are bothering her in her mind are not the ones you need to burden yourself with. Don't feel responsible for the outcome of the relationship. Sometimes things don't work out. Why it didn't work out, will unfold for you when the time comes.
Till then, sit tight, take life by the day. Suggestion - make daily goals. just daily goals. Do what is achievable. Thats it. Just take it by the day, keep your self goal driven and focus on your work/career whatever it is that is keeping you busy. We are here to support you day or night. Just leave a message here and we are here for you.
It is not the end of the world. You were alive and well before she came into your life right? 
