Most Helpful Guy
Don't blame yourself for being yourself and acting in a way who are not comfortable with how you are. From your description, you seem like an amazing person and very fun person to be with- definitely ***not*** boring. I think the main problem are the people you were dating. Too many people want to be on a 'roller coaster' of drama rather than live realistically.
I would say stop trying so hard to find someone, work on yourself, meet new people and surely the right person who likes you for you will come your way. Maybe coming out of shell and being a bit more open-minded would be needed but you will feel the benefits after this. It can be hard work but hey, I am rooting for you!
To directly answer your question. It's not the trait of being 'nice' that's bad. It's the constant shutting out, anti-social behaviour and lack of enthusiasm that is boring to deal with. Stay positive and ignore those who judge you.
Most Helpful Girl
You're probably going to think I'm a bitch when you read the following, but I'd probably take that as a compliment from you. :)
I'll never understand why "nice" would be the highest thing that you would aspire to, or how you would want to definitively characterize yourself. But then again, I'm not the type of woman who goes around calling herself a "nice girl," nor do I really need to use sexist terms to demonize other women as "bitchy" or whatever, to make myself feel good, so maybe that's why. We all want different things in life.
Also, what does being nice have to do with being quiet/mellow? Are these now being used interchangeably?
Anyway, my thoughts. Passive is not something I would consider to be a positive quality (theres a trend here - my tastes are clearly different). Passive to me implies that you don't speak up for yourself, you express many opinions, you literally exist in a passive, non-confrontational manner, which is especially dangerous for relationships in my opinion. A passive aggressive person, for example, allows problems to manifest for years and you never have a clear idea of how they feel or what the problem is. If bitchy is synonymous with having opinions and being assertive about how you feel, then I would definitely say that is me. And I would much prefer that than someone who is passive.
This reminds me of the nice guy syndrome. Nice is not necessarily good, first of all. At least not in my opinion. I've known plenty of people who seem nice on the surface but were actually nasty people or simply not what I personally find to be a "good person." Being a good person is so much more. There are other qualities in your personality other than being nice that are important. The bitchy girl may have those qualities. Who knows?
I would say try to be more than nice. Try to be inspiring, interesting, passionate, intelligent, complex, captivating. Don't just sit there and laugh and cheer people on. Show what makes you unique and interesting. Live. I would never date someone just because they were "nice and cheerful." What else? My time and life are valuable. Investing too much in someone who is merely nice, let alone someone who also values being "passive", is just not worth it.
Don't lose hope though - there are guys who do love girls who are like that. You just aren't meeting them in the right places, I guess. They're out there, trust me. Boring people have their matches everywhere, because not everyone finds the same things to be boring