You know your girlfriend better than almost anyone else, so you may be able to predict how she will react. Some of the ways women react to being told the relationship is over is to cry, have hysterics, scream at you, throw things, and storm out. Unless you perceive that there is any chance of real violence, you just have to put up with whatever scene she causes. That's the price of breaking up.
You can't control her, but you can control your reaction to her. For instance, when she cries, you can start crying too. That will really throw her. It doesn't have to be a sloppy cry, but a tear or two might stop her in her tracks.
If she starts yelling, you can take it with stoic silence. If she wants to know why you are doing this, you can soothe her and lie.
If she doesn't say anything, don't fill up the silence with explanations. Let her finish her dinner in silence and let her leave.
Be prepared for her arguments about why you should remain a couple. She may promise anything if you will agree to stay together. But, by the time you have decided to break up, you should already know that there is nothing she or you can do to change the situation and you are both better off single.
Whatever you do, you should have a disentanglement strategy. Perhaps you can schedule an appointment for one hour after the break up meeting time. If this is an appointment that you just can't miss, you will have to walk out of there composed.
How to Tell Her
Now we get to the nitty-gritty. How should you tell your girlfriend that you are breaking up? Avoid psychobabble. She'll see right through it. Saying "it's not you, it's me" is meaningless because she is going to know that it really is her. You are not calling all women off. You're calling her off.
Be clear that you are breaking up with her and there is no room for discussion. This is a decision you have made and it requires no input or discussion on her part.
Be concise. You don't have to drag it out if you don't want to. She can cry on her girlfriends' shoulders. You are no longer her punching bag.
You may feel that you have to offer explanations, but you really don't. You are not doing her any favors by telling her that she is a nag or that you are tired of her manipulating you. It won't help her be a better girlfriend for another guy. She'll just use your words against you. And, you won't get anything out of it.
Perhaps the best line is "I don’t think we're a good fit anymore." Leave it at that and walk away if you're smart.
Ps. Whatever you say or do in the middle of a breakup it all come down to one thing. Everything we give out
we receive back; the good and the bad so before you are in the middle of a storm learn how to handle it or to avoid it by watching this!