If you and your girlfriend are talking LESS after sex, that's a very bad sign. It sounds like something about having sex with you is bothering her and she can't bring herself to talk to you about it.
There is no way to make her. All you can do is present the problem to her: Tell her that your relationship is getting worse since you started having sex, and that is counter-productive. Tell her that if she feels like she had sex too early that it is okay if you stop, she is not obligated or otherwise required to have sex with you if she does not want to. Assure her that you will never pressure her into it.
She still carries some idea that talking about sex is somehow "dirty" or "bad", and that is it okay to have sex but not talk about it. Her conscience is eating at her, so take the sex out of the equation by stopping it and give her some time to cool off. Spend time with her but don't have sex, don't even talk about it. Just spend time with her and help her relax.
After a while, maybe a few weeks, even a month, tell her you'd like to talk about how things were going bad before. Ask her if she'd feel liek talking about it sometime, and let her decide when that is, don't ask her to talk about it right away. You are both still very young, and emotions are very difficult to get around to talk rationally and objectively, so be considerate of her emotions and give both of you time to de-stress.
Be patient with her and adopt a hands-off policy so she doesn't feel pressured. Assure her that you want to spend time with her because you enjoy being with her, not so that you can get sex.
If you have any further questions, my inbox is always open.