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. 

If sounds like the first thing you need to do is get into good communication with your dentist as to what you are unhappy with and find out what he suggests and is willing to do.  Sometimes a crown can be retrofitted very successfully on a tooth where an opening has been made in the crown in order to do root canal therapy, Obviously proper position and fit will be critical to the longevity and esthetics of this crown. 

Quite often, retrofitting crowns can work and there isn't an issue.  In your case, that is not true and likely simply needs to be remade.  This time, ask for a bonded crown and not a cemented crown, as they stay on better.