Swimming and glucose levels during training

Hello!

I'm going to swim 1 hour 2 times a week.I swim between 1200-1700 meters according to the day.The problem is that I fail to control the levels before, during and after training.I carry the freestyle sensor.There are days that I enter with 180, I like a banana and I finish in 100. Other days I have entered in 200 I have also eaten a banana and I have finished in 250. Another day between 250 and ended in 80. Today I have entered 130,I have eaten a banana and a mejdoul dates, at 20 min was at 115 with a tendency to go down, I ate another dates and 3 glucosport pills andAlmost at the time I was in 90 and going down, I ate another dates and 3 pills, but at the time of swimming I was in 200 ...

Can anyone give me councils to work for swimming and having stable levels?No declines?What do you eat before or during training?Thank you very much 🙏🏻🙏🏻🙏🏻🙏🏻🙏🏻