I echo everything Bibi said. I stopped actively collecting ponies for a long while when I was about 23, but I didn't sell them. They become harder to get as time goes on, and replacing them in the future may not be possible.
If it's 100% what you want, though, and you have other aims and goals to focus on for which the money would be useful, then I wish you all the luck in the world. It's just that parting with your collection is something you have to 100% sure you won't regret.
Maybe keep a few favourites as a memento - ones that hold special memories