Hi Diorlover, and welcome to Scarleteen.
I'm sorry to hear that masturbation has been feeling stressful for you. First off, in case it helps to hear it: it's very common for people to take a while to figure out how to masturbate. There isn't one way that works for everyone, so there's generally going to be a bit of a trial and error process to it, and some folks have an easier time figuring it out than others. If it winds up taking you a while longer to find what works best for you when masturbating, it doesn't mean there's something wrong with you or that you're doing things wrong; it's just part of the process. I know that might not make it much less frustrating, but it's something important to keep in mind.
Anxiety can definitely make it harder to really pay attention to your body, but I'm wondering about the other ways you're feeling when you masturbate. Are you masturbating when you're already feeling pretty aroused? If you're trying to masturbate when you aren't already in that mood, it's likely to be more difficult.
If you haven't looked through this article you may find it to be a helpful resource as well:
Going Solo: The Basics of Masturbation
Also, just to clarify: you say you're 13 in your post here, but your profile says you're 14. Can you let us know which is correct?