The moment that you realize the person you love is in love with someone else your whole world is turned upside down. Everything you once thought you knew, or everything you once hoped for is no longer true. Now you're left to pick up the pieces on your own and watch someone you love, be in love with someone else. And let me tell you, it sucks.
The pain will be excruciating at first. Worse than the tonsillectomy you got when you were 18 and your parents shorted you on your pain meds. You will feel like your whole world just came crashing down around you and that it will take years for you to build it back up. You're going to feel like you will never be okay again, but that is just not true.
When you realize that the person you love is in love with someone else you might blame yourself and feel defeated. You will begin asking yourself questions like, was I not good enough? Did I do something to make them want to find love somewhere else? No. Them falling in love with someone else almost always has nothing to do with you.
Then comes the denial. Your friends might think that you've started to move on because you don't cry every time you hear their name. But nope, it's worse. You have now convinced yourself that this is just their way of realizing that they are supposed to end up with you. Your subconscious will do this to you as a way of trying to cope with all of the pain you've been feeling. It wants you to stop hurting and the only way it knows to do that is to convince yourself that none of the pain is reality. But it is.
Realizing that the person you love is in love with someone else is something that no one ever wants to go through but unfortunately almost everybody does. It will take time but eventually you will heal, we all do. Someday you are going to meet someone else, and you are going to fall in love with them. And this time, they aren't going to leave you for someone else. They are going to love you back forever. You will look back on the time that you had to watch the person you thought was your soulmate fall in love with someone else and you will be grateful that you had that experience.