Try to remember that this isn't your fault.
Chances are if you're feeling sad about having lost your best friend, you were more than a friend worth having. Don't try to count the reasons for this. Sometimes, people decide to go their own way and choose not to look back.
No one deserves loss without even a single reason.
Don't be afraid to make new friends. It's hard to trust when your trust has been broken, especially when it's been broken by the one person who you thought you could count on. It might take some trial and error, and you might not feel like you've found the person to replace her, but try to focus on surrounding yourself with positive people.
Try to face the truth, for you.
Maybe you feel painfully alone. You probably feel angry, confused, and sad. In the end, remember to applaud yourself for the effort you gave your estranged friendship. Accept the fact that your best friend was unwilling to reciprocate. Appreciate the good times and don't be afraid to remember the positive memories, but move on so that you can be better and happier. The last thing you want to do is force a friendship that exists in the past. You will end up hurt more in the end.
Healing does take time.
No matter what you do, you will need time to heal. Losing your best friend is undeniably bound to leave you with a broken heart. After all, this is the person who has promised to be there when everyone else falls short. Focus on yourself to heal. Do your favorite things. Be your own best friend for a while. Take a break from social media and pursue something new. Regaining trust doesn't happen overnight, but you will do it eventually.