I'm in a similar situation, albeit I only run half-marathons and bike up to 60km.
When I started swimming I had the exact same problem, that I couldn't breathe according to what felt a good swim effort.
In the end I relaxed my swim speed as well as allowed my self to breathe every time I pulled back with an arm. I really focused on my breath and feeling good in the water. Everything else was secondary to being able to breathe calmly.
I train in a group, and before I would have to take a break every 50m and wait till my breathing came from a frantic gasping to a relaxed in-/exhale as while running. After the change I swam slower, but could actually manage 1h30 of training, where previously I would be at the end of my strength after 30min. Now, a few months later I can keep up with my group, and I breathe every 2 arm pulls, sometimes every 3 to switch breathing sides.