Most sex educators would agree that knowing our own sexual desires is an important part of sexual health. Simply having a desire or a fantasy is not the same thing as wanting to act that fantasy out in real life. For many people it can be perfectly healthy and safe to explore even extreme desires and fantasies through masturbation. If you are someone who feels distressed by your sexual desires, or out of control, then talking to a mental health professional is probably a better way to go. But if you just aren't sure where your limits are, using fantasy and masturbation can be a great way to map out those lines and understand what it is that turns you on.
Masturbation can be a safe place to check out the less socially desirable sexual fantasies you may have, and see if they work for you. Remember that what you fantasize about sexually is not the same thing as what you consensually choose to do in real life. Many sex educators and therapists would say that any sexual fantasy is okay, providing that you don’t carry out sexual behaviors that are harmful to yourself or others.
So don’t just let your mind wander to the familiar scenarios while you masturbate, try and push your mental powers a bit (using sex toys and dirty talk can help) and shine a light on the corners of your sexual fantasies that you might never share with others.
There isn't one way to do this. You may need to start by developing an awareness of when you are censoring your own thoughts. If you catch yourself thinking of a scenario and then it disappears from your mind, try to pay closer attention to what happens just before it disappears. If you're aware of actively ignoring or fighting off certain scenarios, ask yourself why. It might mean giving yourself permission to "go there" in your mind.
Finding your sexual limits can be a very healthy part of our ongoing sexual development. Knowing where your limits are is one of the best ways of knowing where you want to go in real life sexual situations. It can also help you communicate with partners about what is and isn’t okay for you in a sexual scenario.