The problem may not simply be the fact that you're nice, I suspect it's something else. Each woman wants something different (just like every man, in fact). Some of us want a nice guy, some of us want someone more harsh. If 'harsh' just isn't you, there's no point in trying to be like that.
Remeber that there's a difference between being nice and being a pushover. I'll try to explain: my other half is lovely. He's definitely a 'nice' type. However, I'm often comforted to know that he can stand up for himself if need be, and doesn't need me to do the defending for him. In short, he looks like he can look after himself.
My first impression of him was that he was a nice guy. I liked that, but it wasn't anything particularly special on its own. I might not have taken any further interest in him if I hadn't got to know about his stronger side. This is what happened: he'd just been made redundant but, rather than being cut up about it, he developed a genuine joi de vivre (sp?); he'd been chugging along in a job he hadn't enjoyed and finally felt free to turn his life in a new direction. He was envigorated by this - and I got a strong impression of that from him. He looked every inch the survivor!
When he got a new job it was a rubbish one - packaging sports equipment in a warehouse. He didn't particularly enjoy it but tolerated it because we were planning to move in together by then and getting a proper job didn't seem worth it for the small amount of time he'd be living in that area. One of his colleagues was a tall, broad thug who was smug about the fact that he could beat anyone else in that warehouse up if he wanted to. And he delighted in telling others so.
Mike - my partner - tolerated the oaf's casual threats for a while. So I was incredibly surprised when my mild-mannered beau finally got fed up of this oaf, pinned him up against a wall and told him to 'get fucked'. The oaf was stunned, and never bothered him again.
Now, Mike is slim, mild-mannered and, to the casual observer, isn't one for a fight. In fact, his nickname in several of his previous jobs had been Smiler, for his nervous grin. But actually he's far more empowered than that when he has to be. And that's one of the things I love about him!
So it's not that no woman wants a nice guy - in fact, that's the other thing I love about him. He's gentle, sweet and considered. It's more that, for the woman who wants a nice guy, she wants someone who'll treat her kindly but will also defend himself, and her, if that's what must happen.
Mike is, at heart, a diplomat. In fact, we're having a bit of a problem at the moment and his diplomacy, for once, isn't working. We've increasingly had problems with Mike's parents over the last three years. They expect us to bend over backwards for them, but they won't do the same for us in return. I won't bore you with the details but Mike and I have both agreed with eachother that it's getting unacceptable. A couple of weeks ago we met his parents for a day trip and, at the end of the day, I got so fed up with his dad that I made a comment about something he's done in the past that annoyed me.
I don't do this if I haven't thought it through first - even if I get angry I've still got a very strong 'safety-catch' for having a go at others - but I did so on this occasion because I knew that Mike has been planning on raising several issues with his parents.
Since then we and his parents have phoned and emailed eachother about some of these issues, and Mike, who's worried about disagreeing with his parents, has aquiesced to their demands. In effect, as it currently is, he's apologised for my outburst, allowed his requests to fall by the wayside, and taken on board yet more demands from his parents.
I'm not pleased that he apologised for me and I've been doing my best to be as neutral as possible over the issue while encouraging him to speak up for himself. I've even said he doesn't have to raise any other issues relating to me, he just needs to defend himself to his parents.
He's worried about doing that and is gathering himself at the moment to write an email to them at the weekend. At the moment I feel like I'm sitting on my hands: this issue isn't over until he tells his parents what we both need and he's not done that. I don't want to be the bossy girlfriend to him so I'm trying not to do that, but at the same time I'm thinking, "come on, babe, don't leave me in the lurch like this!".
So you see, I hope this shows why a nice guy who defends himself is so important to a woman: if he wasn't the type to defend himself this whole issue would end here and I would look like the villain. It's threatening to, as his parents are the two people he's scared to argue with, but he needs to say something to them. This problem's not going to get better until he does.
I hope this helps illustrate my point.