My tooth crack down by the gum line and it feels like my back teeth is sinking into my gums. Any suggestions?

It sounds like you may have decay in the tooth and your teeth are shifting in the position of the loss dental structure. Get evaluated as soon as possible as that may cause further problems with your bite which may require more treatment time and expenses to correct.

My suggestion is to see a dentist right away.   You could have gum problems or worse.  Please see a dentist so you dont lose your teeth.  I don't want that to happen to you.  I hope that helps!