I normally think that these kind of things happen for a reason. So I would think that it was the destiny of your horse. Accept it with your real emotions, get over it and you may want to buy another horse to fill the empty space within your heart in a certain way.
I recently had to put my horse down do to colic, it is horrible and was so sudden. I tried to keep busy but it is hard when all you can think about is your horse. I gave myself time to let it out, I cried I starred at pictures if her for hours but eventually I started to see a light. Everyone deals with loss differently. For me the best way was to deal with the problem straight on. I watched videos of my first ride on her and re lived our whole time together. I know it sounds like it could make it worse and it did, but it gave me the time to throw all my emotions into it and feel what I wanted. Eventually I was at peace with her passing, she is not in nearly as much pain as you saw her in that day and that is something you have to remember. Colic is most likely a reoccurring thing after the first time and she never has to go through that again.
I am so sorry for your loss! Allow yourself to grieve, and be kind to yourself. As Lucy said, it's perfectly okay to be upset over this
I understand how devastating it is to lose a horse. It really is like losing someone in your family and naturally it will take a while before you have come to terms with her passing. Keeping yourself busy, such as studying, doing what you enjoy, getting out and about really helps as it can act as a distraction from focusing on the loss of your horse. Don't bundle up your emotions, too. If you need a moment to yourself to cry it out, there's no shame in doing so. We're all human with emotional feelings and sometimes it is much easier to accept that they've passed if you get out all of your feelings and admire the wonderful times you had together. The worst thing you can do is focus on what you can't do. Now you need to focus on what you can do next and what you plan to do equine wise in the future, with the knowledge you received from your horse.
It will get easier - I promise. Keep yourself busy and look forward :)