As Wendy said, it is a simple fact, when your body is used to a tight hand (I assume with lube) then a vagina is not able to compete, plus since you are kind of young and I guess you have watched a good amount of porn and jerked off with that, this means your penis and mind are used to that kind of fantasy,
i experienced the same thing...
honestly, compared to this fantasy real sex can suck!!!!
(specially if you have a strong imagination and think of real images during jerk off or you watched porn every time you jerked off)
it just takes some time and by clearing your mind and coming to the real world you can make it to normal.
This is much more intense for women who jerked off long time before having any sex, it makes their body getting used to the fingers, they can not cum with out finger stimulation of the clit, and many men hate that kind of women, as a result relations do not work well.