I don't understand why you would enter a relationship fully knowing that he's "A nice guy", and then break his heart because he's too nice.
Most Helpful Girl
Most Helpful Guy
You're a nice boy.
What women want is a nice, emotionally strong -man-.
Imagine a dad who gives his kids whatever they want. They ask for candy every day for dinner, so that's what he gives them. Is he a good dad? He's trying so hard!
You probably don't expect to treat a girlfriend like a child you have to parent. And you don't, quite.
But thinking of it that way might help start shifting you in the right direction.
Treat her well. Treat yourself well. Don't give in to tantrums. Make her feel safe by virtue of the fact that you don't emotionally fall apart when she acts up. That makes her feel like you can take care of her, because you're tougher than she is, emotionally.1