If you notice that there is something wrong with your relationship, you wonder how you can show her how to be nice. You must act fast as she may get fed up and decide to end your relationship.
First thing you should do is read a few books on relationships to guide you on how you can handle the situation. You need to treat the woman in your life with respect. If you are living together, then do not expect her to do everything around the house. Sure, she may be a better cook than you are but that does not mean that you can not do dinner some evenings. Or you could let her cook but be the one to set the table and tidy away afterwards.
Some men have a gift for instinctively they know how to treat a woman, but for the rest of us it can be a little complicated. Until I read about some techniques. My girlfriend was amazed. In fact it almost backfired as she thought another woman was teaching me how to behave. Not the result I was hoping for but thank goodness once I showed her my latest reading material she believed me. In fact she was very impressed I cared enough to learn the skills. Prior to this, I had not been showing interest in our relationship . I thought I was doing everything right, I took her out to dinner, brought her flowers and some jewelry but according to her I had forgotten how to listen. And that is more important than my over the top gestures!
Obviously it will depend on the situation with your relationship. If you are asking how to be nice to your girlfriend you are obviously worried that you are missing something. Why not try sitting down with your partner and ask her how she is feeling. Ask her if she is happy or is there something wrong. Some women like gestures whereas others, like my girlfriend simply wanted my attention. I found out that women like to chat even if there is nothing important to say. They actually enjoy talking to us and listening to our views so long as we also listen to them. But they hate when we try to fix every problem. Just because they tell us they are having an issue at work or with a friend, does not mean that they want us to solve it for them. They may just want reassurance they are handling the situation properly by themselves. They may simply be using you to validate her action.
If you want to be together for the rest of your life, you need to learn how to communicate properly. Then you would not be wondering how to be nice to your girlfriend.