The discord between partners with higher and lower sex drives is a very common problem, so take some comfort in knowing that you are not alone.
You have a high sex drive and for you that appears normal. You want to have sex with your partner very often. Your partner, however, does not have as high a sex drive as you do and for her that probably feels normal. The primary problem is the disconnect between your two ideas of what is baseline and normal. It's very easy, especially in an intimate relationship, to personalize issues, as you are doing when you assume that your partner's lower sex drive is a rejection of you. The way to get around this is to open up the lines of communication.
You've been together a year and a half already and it's high time to talk about the basic expectations and parameters of your relationship. In an open, non-confrontational way it would be a good idea to sit down and talk about what it is your both want and need in the relationship. Right now, you're worrying about what-ifs and letter your own thoughts take over. The best way to find out what is really going on with your partner is to ask her, but ask her when you're calm and open to what she has to say not when you're feeling insecure and worried. This will allow you to initiate the negotiation and compromise that all healthy relationships are founded on.
Another thing to consider is to determine whether or now your high sex drive or her lower sex drive are in fact normal. Is it possible that she could be suffering from depression or that perhaps you might be suffering from an anxiety driven problem that the new DSM V is classing as "hypersexuality?" If either of these are a possibility, then it might be worthwhile to consult a mental health professional to explore these issues.
I hope this helps!
Dan