How long have you been together?
What is her reason for wanting to move on?
What is her age and your age?
Any other information that may be helpful?
Well I have some good news and some bad news.......typicaly a women over 40 tends to know what she wants out of life and learns to adapt really quickly. The good news is this can work for you and against you. It can work for you by her changing her mind after a few weeks or it could work against you that she is being honest and simply knows she is not going to change her mind.
The reality is if she is wanting out there is nothing you can do to change her mind.....and at the end of the day do you really want to try and force a relationship? Of course the answer is no. The good news is there are things you can do to increase the odds of her coming back when and if she decides you are what she wants.
The first step is to be honest, straightforward and calm. Do not panic and push her or she will run. Let her know how you feel about her and what you want from her. Let her know that you understand her needs and will give her space. Ask her if she would consider a small break instead of breaking it off all together. In other words separate for a month or so then try again. If she refuses then accept that........do not push or appear desperate. Let her know you accept her decision and should she change her mind that you hope she will give you a call.
The next step is to follow her rules........meaning do not call her, do not text her and most of all do not go to the same places she goes! Otherwise she is going to feel like you are pushing her.
Wait a month or so and send her flowers.......make sure you do so at her place of work if she is employed. Nothing like fresh long stem roses so all the co workers are jealous. In the card simply put "Was thinking of you.....if you would like coffee sometime call me": This will leave the door open for her. If you have followed the above steps and gave her the space she needs then this will be her chance to come back if she really cares for you.
In the meantime move on..........I know this is not easy but if you spend your days moping you are going to miss her even more and may make the mistake of calling her or seeing her and blow what little chance you have all to heck! This will also show her that while you still care about her you are not so insecure as to wait around on her call. It will force her to play her hand as well. She knows you are a good catch.........and the longer you are on the market the better her chances are of someone snapping you up. Nothing like a small bit of competition to get someone in gear. Not that I am saying that you should flaunt it.......never try and make her jealous. The fact is she will either learn it from co friends or may see you in public. If you do see her remain calm and cool....if she asks what you have been up to, be honest. (Just do not flaunt it). Mention a party or two and say you are doing great! The more confident you appear the better your chances are of showing her that you are stable.
I know you want a sure fire way to bring her back to your arms but the reality is that is not where she wants to be right now. The best way to send her running in the opposite direction is to push her. Right now you need to try and give her the space as well as show her that you are a great guy.