Your relationship seems to be a bit boring and stale lately so perhaps you are thinking that if you are more of a sweet girlfriend you can spice it up a little. Some relationships unfortunately just can't be saved, but others can. The point is that sometimes you may try hard to make it work and get no-where and you need to realize that perhaps the relationship just can't be saved. It also takes both parties in the relationship to work at fixing any problems; one of you can't do it on your own. If you are the only one trying to make things work then your boyfriend may just be using you as a door mat. If he really loves and appreciates you then he will put in some effort too.
If you feel that you are to blame for your relationship becoming stale because you haven't been as attentive to your boyfriend or you aren't being considerate of him, then yes you can make the changes needed to fix this problem. This is quite common in relationships, when you become more comfortable with one another you can begin to take each other for granted. If it is only you who is taking him for granted then you can change that, but if he is also taking you for granted then you need to talk about it and you both need to put in an effort to change.
There is no one answer to learning how to be a sweet girlfriend as every girl is different and every relationship is different. One thing you should do is to treat your boyfriend the way that you would want him to treat you. This is the case with any sort of relationship really and is common sense but quite often we forget this rule. If you want him to use manners when he speaks to you then you should use manners when you speak to him. If you want him to call and let you know if he is going to be late home then you should always call him and let him know if you are going to be late. Always treat him the way you want him to treat you.
Another point is to respect his wishes and his views. Everyone has their own views and their own wishes and just because you might not think something is important doesn't mean you should disrespect his view. Often you will agree on situations and sometimes you won't and it's alright to disagree but you should never be disrespectful of his opinion. For example, if he prefers that you call him to let him know you are going to be late, but you don't think that is necessary, you need to respect his opinion because he does care about it. It really isn't that hard to call to say you'll be late, so why not do it if you know it's important to him.
This doesn't mean that you have to do everything he wants and change your whole lifestyle and ideas to suit his; it just means that you need to be respectful of one another. All relationships will need to make some compromises and being respectful and willing to compromise is what makes a relationship work.
It isn't difficult to be a sweet girlfriend it is really all about being considerate and affectionate and have open communication. If you both love one another this shouldn't be too difficult, but no relationship is perfect. Be yourself, but be considerate of who he is too.