Masturbating will NOT make your vagina opening any bigger.
You need to use a lot of lube! Dry fingers are painful! Don't try putting in anything bigger than a finger if it hurts. Stretch using just one finger, while getting clitoral stimulation.
Have you ever orgasmed from clitoral stimulation? G-spot stimulation doesn't do boo for most women so don't even worry about it.
Also, the inner walls aren't your issue, it's the entrance that causes pain. The inside walls are ready to expand to the size of a baby's head!