When you first started swimming in the open water you quickly found out that it is not exactly what you experienced in the pool
. There is a whole new set of dynamics that must be dealt with, especially if the swim is in the ocean. There is anxiety of doing something new and different, to finding you can not swim the way you did in a pool.
Controlling your anxiety is something that can be done by learning to breathe properly. You need to make sure you are breathing in a relaxed manner and dont hold your breath.
Do not focus on the negative things you perceive are going on around you such as the waves, cold water or other swimmers.
Focus on things you can control like your arm stroke, your kick, how your hands enter the water and so forth. Concentrating on these items will help you focus and take your mind off what has created your anxiety.
When you swim in the pool you can watch the line at the bottom to keep in a straight line. In the open water you have to look ahead to your destination to sight in. You will also find that you can drift left and right so you must compensate for this. One way to overcome this drift is to learn to breathe on both sides.
Spend some time in the pool practicing breathing on both sides as you swim so it will be easier for you in the ocean. Make sure you breathe and when you exhale, make sure its in the water, this way you are ready for your next breath as your head comes up again.