Awesome! I love these questions! I think it's great that you're trying to lose weight. Not many people understand that they do.
So stamina and endurance are tricky things to develop but there is a surefire way to make this better for you!
1) Develop a plan. Take a calendar or a sheet of paper and establish the days you are going swimming to begin with.
2) Take each day you're going to go swimming and say "alright, on the first day, I'm gonna do one lap. Same with the next day. But the day after that, I'm going to do one and a half." So slowly and surely build up the amount of laps you can do. BE CAREFUL. Go slowly. Do one lap for two days without stopping. Then do one and a half laps for another two days without stopping (or stop just once). You don't want to overwork yourself but it's crucial that you build up. Also, stick to your schedule. Do not wimp out! YOU KEEP FIGHTING! You can do this!
3) Eventually, as you keep building up, you'll find those first two laps becoming easier and easier. Then the third lap becomes easier and easier. And before you know it, not only have you lost weight, but you've also built up your endurance more than you can imagine.
The trick is to stay determined and fight through. It's not gonna be easy, but if it was, everyone would do it. Be careful, stop if you have to, but try to push to a healthy extent- that's when you lose the weight.
Good luck :) I think you're awesome for trying to lose this weight!