Perhaps she doesn’t have very much experience with relationships, perhaps she has been brought up with conservative values, perhaps she has been emotionally hurt in the past by rushing into things with a different guy, or maybe she has a different reason, but either way, when it comes to her relationship with you, your girlfriend wants to “take it slow.” While it is possible for you to figure this out by implication – that is to say, from her actions (or lack thereof) – it is more common (and best) to figure this out through her telling you she wants to take it slow.
This can mean different things within the context of your relationship. It can mean she is nervous about a specific “stage” that your relationship might be approaching; she might be anxious about you moving in together, about you meeting her parents, about getting engaged, or other similar things. It could mean that she is nervous about having sex with you, and wants to get to know you really well and be really sure about the person you are before doing that with you. Or it can even mean she is not sure if she even wants to be exclusive, or public, about her relationship with you.
Whatever it does mean, if you know that you like her and want to be with her, the fact that that your girlfriend wants to “take it slow” can be extremely frustrating.
If you’re already in a relationship with her, and want to meet her parents or friends, or move in with her, or even ask her to marry you, it probably means you’re certain of how much you like her. If she is hesitant it can simply be frustrating, as it is delaying the thing that you know you want. It is also, in this situation, fairly impossible to compromise. The person who wants to take it slowest almost always gets their way.
If the problem is that she doesn’t want to sleep with you, you have a different issue. This is, of course, frustrating in its own way. You are obviously attracted to her and her to you, and the desire to express your love to her physically (and of course, to experience the great feeling of sex) is probably very high. The worst example of this is if she decides she does not want to have sex before marriage, something that can absolutely define a relationship in a way that you might very much dislike.
She might also simply be anxious about going public with her relationship with you. This can in many cases be the most frustrating; it is difficult not to take it personally if the problem she has with your relationship is anyone knowing about it. Sometimes it might simply be because she has conservative parents or something of the sort, but it is still difficult to separate completely the notion that she is afraid you will make her look bad.
As annoying as it might be, if you’re in the situation where your girlfriend wants to take it slow, you pretty much have no choice have to be patient with her and not pressure her into doing things that make her uncomfortable.
If you care enough about her, it will certainly be worth it. However, you should try and communicate with her as honestly as possible about how you feel, and hope that it is something you can overcome together and not something that will eventually drive you apart.