I think you need to challenge those thoughts.

First, you need to identify why you want to self harm. If you can't identify the reason, it will be impossible to challenge those thoughts. So you have to ask yourself, "Why do I want to self harm?" and "What makes me want to do it?" Then, once you've identified the reasons, ask yourself if they are true/justified (because 99% of the time we get stuck in a vicious cycle where we aren't being rational, so we have to logic our way out of it). If you've done this correctly, you will find that they really aren't justified or true. It may also be helpful to consider how self harming impacts you in the long term. Is it a *safe* solution? Are there better ways you can cope?

P.S. @fern46 posted a helpful "self care" thread where you can read about some self care tips