You have two questions:
(1) "Why are women/girls so rude?"
(2) "Am I doing something wrong?"
The answer to the second question is: Yes, but there is no indication in your text exactly what you are doing that would affect the attitudes of rational associates. If they are not rational, it is outside of your ability or anyone else's to influence the reasoning from which they act--they are not reasoning.
The answer to the first question is irrelevant since you have already contradicted yourself and it cannot be taken seriously as written: "Not all of course, but all the the women I have met. I'm a 20 year old female..."
If all the women you have met are rude--whatever that means--then you cannot know whether or not all women are rude. So how did you come to that conclusion? It is a non sequitur:
"Non sequitur is a Latin phrase that means 'that which does not follow'. It means that the conclusion reached does not follow from the premise(s). Often examples of non sequitur arguments are hilariously disconnected, but those encountered in the wild can be subtle and may not be easily uncovered. The reason that such arguments are fallacious in logic should be fairly obvious."
• http://rationalwiki.org/wiki/Non_sequitur
The remainder of your text ending with a request:
"... I'm a 20 year old female, and I have no female friends, only guy friends. im straight, and I don't sleep around so don't tell me that's why I have no female friends... This is going to sound conceited, but it adds to my question. All of my life Adults have always made a huge emphasis about how nice/sweet I am. Also, I used to be an uglier child but that's when I had the most female friends. As I've gotten older and started to care more about my appearance, I've lost literally all female friends. Now I can't make any. I will start to get along with a girl and she will end up ignoring me and acting rude. I'm not clingy. Like I said, I'm nice (I think and that's what people tell me). And I'm definitely not cocky, in fact I'm more on the low self esteem side, but I don't let it show... My parents will ask me how it's going with my friends, and I'll always have to tell them that girls have stood me up once again. But my guy friends are super loyal and just awesome. I'd just like to have like 2 female friends to go shopping with or something... Please help. I just need advice"
I suggest that you determine why you want the attention of other females. At your age it is not exactly normal for women to go out and seek other women to praise, ditto for men your age, so you ought not to expect it.
That does not mean it is abnormal for people to have same sex friends and enjoy spending time with them.
Among human beings there are often time constraints that they must abide by in order to make a living and still get what they want biologically or career-wise.
You may be coming up against that situation in regard to the selected women you know.
Naturally, men will be more interested in you than will women. Human beings have a biological propensity to seek mates, even if they have no clue as to why or how it is that having sex leads to making babies.
It would be far more realistic to focus on what you can do for your own mind and life rather than seeking absolute positive regard from grown women. Each human being is unique and they have their own problems, their own personalities, their own sentient experience, their own goals.
There is some probability that you have just chosen rude people by random occurrence. That would be a long shot--although it does happen, for example, within cliques with a shared social cognition schema by which members have a certain amount of contempt for outsiders--by whatever metric, of which there are many ranging from ethnocentrism, to political affiliation, through mind control cult loyalty.
Those would be people you ought not to associate with. Nothing good can come from them.
But since your metric for rudeness is undefined in your post, there is no way of judging from your text what your situation is in regard to group dynamics.