Me and my girlfriend of 1-year used to have sex 4 to 6 times a week. But now she says she does not get in the mood and we usually only have sex like once a month. I ask her why she just says not horny anymore.
Well, it's important to remember that bodies are not machines and they're not exactly the same all the time. Just like many other things in our lives, our desire for sexual activity can wax and wane over time. In other words, everybody goes through periods where they may be interested in sex (of any sort) more and periods where they just aren't as interested. Lots of factors can cause these fluctuations. For example, stress, diet/exercise, lack of sleep, major life changes, some medications (including things like hormonal birth control and depression/anxiety medications), relationship changes, regular old hormonal changes, and many other things can impact our libido at any given point. Many times, we won't ever even figure out why we have periods of low interest in sex...it's just something that comes and goes.
There are a couple of more pertinent issues here, it doesn't really seem like you're satisfied with the amount of sexual contact you're having right now. Is your partner wanting more or less or is she satisfied with what's going on now? Is she enjoying the sex that you are having? (In other words, when you do have sex, is she expressing enjoyment at what is going on?) If your partner is not feeling good about what's going on right now, then that's something to address. First off, she can head to her doctor and address any physical issues that might be going on that could be influencing her interest in sex. If she is on any medications that can change one's ability to become aroused, then she can talk with her health care provider about maybe changing what she is taking/doing. If she's having extra stress, then she can take steps to address those issues. It's also sage to check out the health of your relationship if one or both of you are not satisfied with the sex you're having. Sometimes issues in one part of our relationship can bleed into other parts. Also, if there are issues that one (or both) partners are not talking about, it can lead us to want to hold off on sexual activity or have difficulty with arousal. If she is not comfortable with something or if she's not cool with the sex that you are having (in other words, if it's not satisfying and enjoyable for her as well), then that could definitely be playing into what is going on. Are there other things that she is wanting or needing sexually? Is she uncomfortable with something in the relationship? Talk about your relationship and see how things are going.
You may just also need to be read to deal with this being a bit of a dry spell when it comes to sexual activity. Your partner just may be going through one of those point in life (or in this relationship) where sex isn't a huge priority. And that's something that you will just have to respect. Obviously, it's a good idea to sit down and talk about all of these things. If you want more and she wants less, then it's something to discuss. Find out what is going on with both of you and with the relationship. But if one partner is not interested, there is nothing you can (or should) do to try to force it to happen.
If sex on a more regular basis is a "deal breaker" for you, then you need to decide that and consider whether this is the best relationship for you to be in right now. While sexual compatibility is not necessarily the only thing that is important in relationships, it is something that we often consider when deciding whether to stay in a relationship or not. (Though along with that, I'll tell you quite honestly that you're not likely to ever have a relationship where the sex is exactly the same in amount or activity forever. As I said earlier, we all have those fluctuations and that is something that you have to deal with no matter what partner you are with.)