If my dentist messed up the length of my veneer to match the other front tooth, what can I do to add length or fix it? (photos)

 The only way to fix it permanently is to replace it. However, the length can be tested by adding composite resin to the veneer. This can help in two ways.  It will let you and your dentist see and agree  to the proper length, and if it adding length will interfere with your bite against your lower teeth.  Once the composite is added, and your dentist agrees that it should be longer, he should take an impression with the composite correcting the length.  He should also take before and after photographs with and without the composite in place. In the final analysis however, the veneer will have to be replaced if you want it longer permanently.  

I does look like a different length. It also has a different shape than your natural tooth. Part of the problem appears to be the way your lower tooth bites to it. The photo makes it look like they are edge to edge- making it almost impossible to get the length correct without adjusting the lower tooth.