Most Helpful Girl
It might be that you may be giving the impression of a being a push over. Just show that you're a strong person and you should be okay, being a nice human being is a good thing. A lot of women wish they had a nice guy, the right one just needs to come your way and she will in due time.
Most Helpful Guy
I think "nice" is a made up word, at its real-est, its a euphemism for boring. Someone is nice when there isn't another word to describe them.
So the question for you isn't how to be less nice, but how to be more anything else. You likely lean towards something, reflect about that and other ways people have described you and develop those areas.