I have been very claustrophobic all my life; I have also flown 68,000+ miles year to date.....
I fly United whenver I can, and because I am an elite member, I am seated in Economy + with more legroom, and/or get an upgrade. I try and find less popular times/flights and won't fly on an RJ or anything smaller. This sometimes means driving up to 5 hours out of my way.
If you don't have status with a legacy carrier, be aware that you may not be able to get a seat assignment in the front section. You'll be at the mercy of a gate agent or passenger if that happens. I sometimes have to fly Delta and have no status; once a very kind FA asked if anyone would switch, and a kind passenger did give me her seat and took mine back of the bus. BUT don't expect that most FA's will lobby on your behalf, and don't expect someone to change just because you ask (If you asked me to change out of my usual 6c, for instance, I would have to tell you 'sorrry')
Check seatguru.com for seat maps. If you want to try your luck with SW you may actually be better off, since you can control your destiny somewhat and try for an A boarding pass. (Someone else can explain SW better as I never fly them)
I take a sleeping pill most trips 10 minutes before boarding; due to my elite status I am one of the first on board. I grab a pillow and blanket and get into my seat and immediately try and sleep - by take off I am dozing. I don't try and put anything by my feet except for a small purse, as it makes me feel like I have more space.
I always try for aisles - if not available I'll go for window bulkhead. Exit rows may work for you too. (But no kids under 16 in exit rows) I am usually in 6c on an Airbus 319 or 320, or 9a or 9c on a 757. But again, that's the Economy + zone so non-frequent fliers may not get these seats.
Note that United SELLS Economy + access too - you could buy it for a year, or just hope that it's available for your flight. That way you can almost guarnatee the extra space.
Also avoid any stress before the flight- I have my routine down pat now and since I fly so often it's like commuting (and for me, easier than riding in a subway, etc)
It can be done! Hopefully I can answer questions if you have specific ones....