Every SSRI med is different so your allergy to Celexa should not be a reason to be allergic to another. SSRI are for long term anxiety because it builds slowly in your system for up to 8 weeks before it peaks because your body gets rid of it as fast as the next dose is taken.
Xanax is for short term panic situations so should be used sparingly to avoid dependency.
I am allergic to one anti-bacterial med but all others are totally tolerated by my body. Possibly you would find no tolerance issues if you tried a different ssri.
If you fear concussion effects and are worrying about your mitral issue then it is not surprising you have developed some anxiety issues. " I've become very aware of every little feeling or change in my body. " The over-analyzing of every body function is typical anxiety - you are searching for clues to some mystery illness that you think you might have developed so in your heightened sense of awareness every twitch, ouch or itch becomes a potential "clue" to analyze. Prior to being anxious you would ignore these "clues" because you knew you were healthy and they were non-events but now they are all noted and you wish they would stop, which perpetuates the anxiety issue.
There is nothing you can do about the concussions now and the athletes who suffer as a result of them likely had many more than you do, so it is not a good subject to dwell on. I read that every lineman on defence or offense in the NFL gets a hard knock to the head (hard enough to have a lt effect even if each is just tiny) in every practise or game, so that is likely much more than you were exposed to. I would stop thinking about it because lots of former pros become announcers etc so only a select few suffer. The only hting to do about the "clues" is ot realize that you didn't catch some cocktail of exotic illnesses at the same time, so accept them as likely events that you always ignored in the past and your anxiety level will go down and they will disappear. I am not saying this paragraph's recommendations are easy but in one form or another resolving them is the key to an anxiety free life.
Some people can't ever get rid of anxiety and I am not saying you will be able to, but on the other hand many do resolve their anxiety through counseling or meds. I ended up on the med route and after 6 great relaxing months went off them, but the key to being off is to focus on reality, not worrying about things that aren't real. if you read this you will see a lt sufferer's story.
http://www.medhelp.org/user_journals/show/857117/CNN-article-about-anxiety-sufferer