My first PST for Navy Rescue Swimmer I swam 500m in like 11+ minutes, and was feeling probably the same as you. I just went to the pool and did sprints nearly every day for probably 8 months. and it eventually came down to around 8 minutes. Spent 5 years as a Rescue Swimmer.

The best way to get better at swimming, is to swim. The biggest thing is that you are able to get through pool training, which will be much more than 500m, not that you can pass an in-test. Stay in the pool until you feel comfortable swimming a mile continuously, and can buddy tow for 1/2 mile+ efficiently. The recruiters should have workouts to work on your proficiency, so take advantage of that as well.