The thing is, it's actually pretty uncommon for people with vaginas to masturbate just by putting fingers inside them or find that alone all that pleasurable (just like most people with vaginas don't find intercourse all by itself all that exciting, either). Most people with that anatomy don't masturbate that way for the reasons Sam has already explained.
So, if you want to experience pleasure with masturbation, the way to go is to experiment with what feels good, rather than by trying to make something that does NOT feel good feel good. In other words, as Sam has been trying to explain, it's clear you're really hung up on making this all about vaginal entry, but that's just not something -- be it with masturbation or partnered sex -- that's all that pleasurable all by itself for most people. You're not alone in that.
Do you want to talk more about why you're feeling so desperate about this?
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead