Thank you for that extra detail, that's really helpful!
It's understandable that when you're engaging with a part of your body that's extra-sensitive that you'd worry about it hurting. And it's also totally okay if you need to ease yourself into it, or take a break from trying to masturbate for awhile. It may help to think of it less in terms of masturbating "properly" and more in terms of listening to your body. Usually, our bodies give us little twinges or "ouch" moments as a warning if we're doing something painful (that's setting aside way bigger "ows" that happen suddenly, like dropping a book on your foot). So if you're paying attention to the signals you're body is giving you, you'll be able to stop before it really hurts. Plus, there are things you can do to lessen the chances of pain, like using lots of lube and making sure you're fingernails aren't too long.
If it feels weird going deeper, then it's okay not to go deeper. There actually isn't much back there in terms of sensory nerve endings, so there's very little pleasure to be gained by going deeper, especially if the sensation already makes you uncomfortable.