Oh, I'm so sorry that this happened. I know you feel bad, but it was just an accident. The only problem it might cause is that she may or may not grow a long full tail like a horse with an intact tailbone, it might always be a bit shorter and thinner than others, but thankfully that can be remedied with fake tails if you need to.
Just to give you an idea about the healing process that you have coming, here is a thread where another member here had the same basic problem a while back on her pony. He healed up fine, his tail is just really short now. Please send your positive thoughts to my beautiful pony