Lake swimming

Get as many lake swims in as you can and stop thinking about all that other stuff.  It's only water, and you know how to swim.  So swim in it.

You still need to keep putting in the work in the pool.  It sounds like you aren't freaking out, which is good.  If you continue to become a better swimmer through pool training, you'll find that swimming straighter and your general confidence will improve.  This will help you forget all those thoughts that are creeping into your head while in the lake.

Here's a little clip I made to show people how to sight. I am swimming on a bungee cord but it gets the point across. I usually sight every 3 to 4 stroke cycles and when it's done correctly doesn't mess up your stroke or use any excess energy. I hope it helps.

Open Water Sighting

One other pointer about sighting that I have given is to look for objects that are in the line of the buoy. Things such as big trees, hotel roof tops, farms or silos, radio towers, etc. This way you aren't trying to focus on a small buoy at water level. I felt as though I was cheating in a race last year (Musselman) where there were whitecaps on the water and people were swimming perpindicular to the course. I had focused on a large farm on a hill above the buoy that was there every time I sighted. The more you practice, the easier it comes. Good luck!

Something else you can do is practice sighting in the pool... not looking at the blue line but put your swim bag (that holds your fins, bouy etc) or a large water bottle and stick that in the middle of your pool lane. Every third stroke or whatever look up and site off that object.. just practice it everytime you come up that side of the pool length.

Like other have said, you have to practice sighting. then when you get ok with it, you swim in choppy waters and you learn you have to develope a new way to sight when the bouy you are sighting off is above you on one stroke then is below you on the next breath because of swales...