Sounds like you are making great progress adjusting to the murkiness of the water. If you can do relaxed 400-500m segments, it’s a short hop to much longer distances. But you bring up the all-important psychological side of swimming.
I’ve experience a couple psychological impacts on my swimming over the last year. Being a relatively new convert to TI (working on it for a little over 1 year now), I could swim a continuous mile with my thrash-and-dash style, but when I switched over to whole-stroke TI, I found that I couldn’t swim more than a few laps without going into major oxygen-debt. With some advice from TI-ers Werner and Ananth to slow down and relax, I was able to quickly get back to a continuous mile. That ability apparently was always there, but my technique was holding me back and I had gotten into a mental rut that I couldn’t do it. By relaxing and trusting the TI approach I was able to breathe appropriately and extend my distance accordingly.
Another example is fairly minor but instructive. My open water race was supposed to be held in a fairly deep reservoir, but due to heavy Spring rains the water depth and the potential for debris caused the venue to be moved to a much shallower lake. The reservoir was about 860 feet deep, whereas the new venue was a maximum of 21 feet deep with an average depth of only 8 feet. Even though I knew mentally that a swimmer only uses the top 1 or 2 feet of water regardless of the water depth, it was more concerning to me to swim in the reservoir than the shallower lake. A friend of mine tried to cheer me up by saying, “The only major difference would be that it would take them longer to recover your body in the reservoir than the shallow lake”. So much for the encouragement!
Your use of the Tempo Trainer sounds like a great approach. It helped me a great deal to simply focus on a single stroke, then the next stroke, etc. – kind of like focusing on the next step during a running race. I can tell you are on the verge of a break-through. Keep us informed.