He is right. In your case, that is all you CAN do - nothing.
Well, nothing to "save" this.
.
I have a unique circumstance right now regarding a breakup, but it is different in that it involves a serious illness. Very complicated. But for the most part, I have had to come to accept the steps I am going to recommend to you.
In YOUR case? Here is what I recommend, FWIW.
1. Accept that it is over - that is the hardest part.
2. delete her number and move all reminders of her to a box in the closet. After a couple months, maybe to the dump?
3. Work on yourself - it is all you CAN do! You can't control someone else. In the end, she wanted marriage and you didn't. If you had, you would have acted on it. So, let her go get married. She very well may realize after a few months, that she gave up a gem in you. But you can't wait for that.
4. Go straight into no contact and don't break it. She dumped YOU. If she realizes that was a mistake, she will contact YOU. At that point, you can decide what to do. There is no reason for you to chase HER. Chasing will NOT change her mind, your absence may. She needs to MISS you and she won't if you contact her at all. She nneds to wonder what you are doing, how you are doing, if you are seeing someone else and if you are moving on. THIS is the only thing that will make her reach out (if anything does)
5. Hit the gym hard, spend time with friends, stay busy
6. Realize, that although she was a "good fit" (as mine was) there are OTHER good fits.
Realize, as far as "getting back together" you have no control. She will do what she will do. However, if you move on, focus on yourself, and prosper and she contacts YOU - that tells you she has realized the grass in not greener. She may, she may not. Assume she won't.
All you can do is move forward.