how do I deal with falling in love with my best friend, then them leaving me… I am completely heartbroken
Rejection and loss is always so hard 
It is okay to feel heart broken over losing someone close to you. You will get through this time of pain and heal with time and space
I’m sorry that you lost your best friend.
I think maybe talking to them if you can would be a good idea, to see why the left. However, if that’s not a possibility, you may find it helpful to write a goodbye letter, or speak it out loud. Even though they won’t know that you’d done that, you will still feel a sense of closure. It won’t stop the hurt. That’s something you are going to have to work through and accept, but, it will give you a little bit more of a calmness.
Lean on your other friends. Talk to them. There will be someone else - it just takes time to find them. We are here for you whenever you need us.
Hold Fast
Kayla
I am so sorry you are going through this right now, friend. I just want you to remember that this pain, this heartbreak won’t last forever. It takes time, but this feeling won’t last forever. Little by little the pain will disappear and you will be okay. One day at a time.