New and in need of advice!!!!

I am new this year with Tris, Completed the hillsbough sprint. Have a buddy that does a great job of getting more out of me, but the problem is i have committed to the missionman in july. I have decent endurance, no problem getting on the bike and maintaining 15 mph for 80 miles, running 12-13 minute miles ( I HATE RUNNING) my major problem is breathing with swimminng. I have been told i have a nice stroke but i get this anxiety type feeling and im not sure if i breathing too much, breathing too deep to shallow or what. Example, I can go to one end of the pool and feel great on the way back i get 3/4 down and i get the feeling of not having enough air in my lungs. I really need help! Any suggestions???? And please be honest have i bit off more than i can chew i have a little over 4 weeks left and i am willing to get in the pool twice a day in necessary to finish the distance but is it enough time with the problem im having? i need to finish 750 in an OPEN water swimmm. What am i going to do????????

Funny thing, I use to be a life guard.