First, listen to this - https://www.youtube.com/watch?v=86gqjRoyHrA
I won't get into what happened. Here's just some thoughts on heartbreak.
When you have been planning on spending the rest of your life with someone, it hurts like hell to see them walk away from your life. It hurts so much because all you want to do is scream after them, "I LOVE YOU. COME BACK." But that's a truth that both people are very aware of, and it wouldn't help the situation very much. When everything you want is for that person to hold you just one last time, but then never let go - but you know that would just end up hurting much more in the long run.
What hurts is knowing all the little milestones that will come up in life, you will be facing alone rather than with your partner beside you. All of the plans which the two of you had for your future have all just disappeared and you need to rethink all of your goals and dreams.
Yes, there will be someone else eventually, and they will probably be better and will fight for you as much as you fight for them, but it doesn't help in the moment - does it? Right now, it's shit. And that's ok. Your heart has basically been cut open by the person you trusted more than anyone in the world. Allow yourself to feel EVERYTHING. You are hurting. Your heart is trying to heal, and it never will unless you cry out all of your tears and yell at whatever you feel you need to yell at, even if it's just the sky. You won't feel better immediately, but in the long run - you will have gotten rid of lots of pain, and you won't have stuffed it down inside you unnecessarily.
It hurts, and you will be feeling physical pain. It's funny how our heart does that to us... Make sure you have done everything you possibly could do to save your relationship, so that you can remind yourself of all the reasons why it didn't work out. It will hurt, but the more you remind yourself of the reasons, the conversations, the more real the change will feel, and the quicker your body will allow yourself to move forward.
Lose yourself in the things you love to do. Sing. Play music. Take a million photos. Write a novel. Work. Paint. Make Art. Continue life. And build from the pain. Some of the best songs, books or pieces of art are created when the creator has been in some sort of inner-turmoil. Build from this experience and learn what you need to do the next time around to avoid this pain. Don't stop yourself from thinking or feeling the situation. It's all very important for you and for shaping who you become from this.
Remind yourself of the good times. Think back on the fond memories. Don't allow yourself to get bitter. That's the worst thing you could do.
Talk to people you trust. This is what they are there for. Don't lock yourself away. Tell people you are in pain. They will understand. And sometimes all you need is a good long hug.