How to stop being a "nice guy" and be assertive?

I think you are just working too hard at this. If you acted like you could care less if they went out with you after you ask instead of pursuing them even more you would get their attention. You have to play hard to get. If they say no then move on. Many girls. and guys like you, desire things or people that they can't have. The whole secret to meeting girls is knowing which ones are attracted to you to starts with. Maybe you just need to look around at all the other girls that you are guilty of ignoring, and you will find out you have lots of options. If one girl likes you it gives you validation with the others. It means you must be ok. Don't worry about being a nice guy. Why try to be something else that you aren't. . When you get older being a nice guy will pay off for you. It's all a learning experience.