I doubt that's the cause. Masturbation normally causes you to be desensitized actually. People normally develop DE (delayed ejaculation) because of it.

Being 15, now there's a cause. You're swimming in testosterone and excitement right now and now you've that anxiety in your head which is going to make it even worse. Calm down, relax and don't worry so much about it. You've got a lot more you need on your mind than worrying about PE.

Also, don't worry about a permanent cure until your testosterone production levels out in 3-5 years. Then if it's still an issue in your 20's, you can start practicing "male Kiegals" and work on any psychological things that might be increasing your anxiety about it.