Ok here are some answers:
1) Being promiscuous doesn't make her vagina change that much
2) If you are 18, probably your increase in size was from growing naturally and not from jelqing
3) You have to learn how to fuck. This may sound strange but just sticking your cock into your girl -- or, as you said, forcing it if it is too big -- will not please her, and will hurt her if she is vaginally small.
With this new girl, use a lot of foreplay. That does not mean the time she sucks you. Work on her body, all of her body. Find all sorts of ways to give her pleasure. Touch and lick (especially lick) her pussy. Find the places and ways of licking and touching that give her the most pleasure. Use your fingers inside her to find her g-spot (if you don't know, look it up on the web). See if she loves you stroking it inside her. Make sure she is really open and wet -- one or two orgasms. Now put your cock at her opening and very easily, with short strokes, enter her without forcing. Each stroke can go just a little deeper. See how she responds. See what kind of stroking she likes best.
This is your job. Fucking like you see in porn is gross and will not please her.