Reason why I don't have a SO?

I'm a really sweet girl, I take care of myself and have a really outgoing personality. I feel like the boys constantly take advantage of it. But the problem is, is because I let them. I'll say no and they won't accept that answer (in various situations) and I end up giving in. I want to be respected more and taken seriously. Any tips? I feel like this is a reason why I don't have a boyfriend. 😣