My Crown Doesn't Reach the Gum and Hurts my Tongue with It's Jagged Edge?

There is no excuse for poor marginal integrity on a crown.  Without proper crown margins the tooth in question is ultimately doomed for failure.  Decay and infection will ultimately result as well as probable need for extraction.  Sometimes it is difficult to get a good margin because of location, but ultimately it can be done as long as you are being treated by a competent dentist.  Seek out a second opinion

It sounds unusual that your dentist would say it cannot fit because of where it is located. There are times when certain areas of the mouth are more difficult to obtain a perfect fit however that is not really a very good excuse. I would recommend you seek out a dentist for a second opinion. 

One of the successful criteria of the full coverage restoration like crown is the marginal integrity- that means that the margin of the crown should blend into the tooth preparation line. No matter where in the mouth it is located. Other problem except constant trauma to the tongue will be decay underneath of the crown and inflammation of the gum. This crown needs to be remade with better marginal quality in order to have good long-term results.