Less Than a Year After my Root Canal my Crown Continues to Come Loose. What Can I Do?

The situation that you are describing sounds like you should probably have a new crown made.  The "black line" that you are describing tells me that it is a partially metal crown.  If there is sufficient tooth structure for you to have a new crown, I would specifically ask for an all porcelain crown.  They are more aesthetic and have a higher bond strength than porcelain fused to metal crowns. If it is deemed that you do not have enough solid tooth structure for a new crown, then the best long term solution would be to extract it and place an implant.  Find a dentist that you trust and allow him to evaluate the tooth and go over your options with you.  Good luck.

I think the best thing to do here would be to remove the old crown, re-prepare/build-up the tooth structure underneath, and have a new crown made. Fitting an old crown back on a tooth can be tough, and it's clear it isn't going to work here.

It sounds like quite a bit of dentistry has been done on your front tooth.

First a Crown, then a root canal and then an additional root canal to correct the first one.

Once a Porcelain to Metal crown has a hole placed in it to access the center of the tooth for the root canal therapy the entire tooth and crown have been compromised. 

I think you may need a new post and crown to adequately carry the load that will be placed on your tooth.