Let's face it, some men prefer to have a girlfriend that comes across as being, for lack of a better word, sweet. Granted, the idea of sweetness covers a lot of ground, but you can certainly be sweeter if you want to. We are not talking about being sickeningly-sweet or phony, but rather about having a certain kind of positive attitude.
Before you start making changes to be sweeter, you need to ask yourself why you are trying to change. Are you doing it because you want to make your boyfriend happy? Because you know you come across as a bitter and sour person and have wanted to change for quite some time? Or is it because your boyfriend has asked you to change? You should never be forced to be somebody you're not, so if your boyfriend is "making you" change, then it may be time to change boyfriends, or at least tell him to love you for who you are.
The next question you need to answer is what does 'sweet' mean to you? Think about other people you know that are the kind of sweet you want to be. Pay attention to how they behave whenever you can. Take note of what they say and do. What is it that makes them different? Chances are that they smile a lot, a somewhat demure and are polite. On top of that, they take a genuine interest in people and show they care in a number of ways. You may be surprised at what you observe, but take mental notes and try to do some of the things they do.
To be clear, we are not talking about being somebody you're not. Only do those things that you want to do, and don't let anybody force you to do something you don't want to do. That being said, if you are trying to change for the right reasons, but just can't seem to do it, then you may need to try a different approach. Instead of observing people you consider sweet, you may have to ask them for their secret. You need to do this in a way that doesn't come across as being weird. A good approach is to tell them that you think they are nice and that you are trying to be nicer yourself, then ask them for their secrets.
Another important aspect of being a sweet girlfriend is making your boyfriend happy. Now, that doesn't mean you should wait on him hand and foot, and do anything he requests of you (you are trying to be sweet, not a servant), however, having a cheerful attitude when doing the things you normally do will go a long way toward making you sweeter person.
Remember, how to be a sweet girlfriend has nothing to do with being a pushover. It is simply a way of showing your boyfriend that you care about them, and that they make you happy.