Thank you so much, for really constructive tips! I will try them out. I knew swimming was my weakest point, but I had forced myself to do open water swims in advance, by myself, in cold water, and up to twice the length of the race distance, and was feeling pretty confident than I could handle it. I'm not used to close contact, but training in a pool I had experienced light contact quite often. What I hadn't reckoned on was how my normal tendency to get highstrung before any performance (race, presentation at work, even catching a plane the next morning) would affect my breathing, and how essential controlled breathing is to swimming. On a bike you can just gasp away until it goes away. I think the worst thing was hearing everybody else panting around me, it just "told" me that I should keep the same rhythm 
I somehow have to try to replicate that kind of adrenaline spike, and try to work through swimming with it. The best thing to do would be to start in a tri with the only goal of starting in back and doing the swim with relaxed breathing...
It really helps hearing that this is kinda sorta normal!
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett