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)

You have two choices.  Have your dentist shorten your veneer or have it remade.  Shortening it slightly would be easier . 

Hello,The veneers have different in your shape of it that reflects the large is affecting, I recommended you do the veneers again in case you want to improve the esthetic. 

Hi there.  The only way to permanently add length to the veneer is to remove the one you currently have and to make a new one.  Adding material to the edge of it will not last and can break off at any time.  However, judging from the first photo you provided, if that is your natural bit and your teeth are hitting edge to edge like that, it may not be possible to lengthen that veneer because you would risk it breaking even more.  Make sure you talk to your dentist about this before having it redone.

 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.