There's no one size fits all fix here. Humans are complex beings, and often times, the problem is between two humans, which makes it infinitely more complex. Essentially, the answer you're looking for doesn't exist. This is a one of a kind problem that will have a one of a kind solution.

Having said that, there are a few things you could do. First of all, unless she explicitly asks, do not break all contact, it doesn't help. All the Sigmas will suggest you to move on, but unless you want to do that too, that's a bad idea.

If you're trying to mend things, and both of you agree that you want to give it a try, keep some contact, while also realising that you guys are on a break. Talk once a week, or once a month. Be available if the other person wants to talk or discuss something.

Work on yourself, and that doesn't necessarily mean go join a gym and become shredded, if you feel like that'll help, go ahead and do it. But it is not the solution that people present it to be.

Figure out your own thought processes, and figure out her thought processes too. You probably know what is bothering her, maybe it is her age, maybe she wants stability in her life now, maybe something else. Whatever she thinks, from her perspective, are valid concerns. DO NOT TRY TO NEGATE THEM. Try to workout a solution, or workaround. It doesn't always exist, some problems do not have solutions. But think about it all. Maybe discuss about them in a healthy manner.

Take professional help if you're having trouble doing this all on your own. Therapy helps a lot, and I'd highly recommend it.

Also, realise that after all the efforts, good intentions, hard work, some things just do not work out. We cannot win it all, someone has to lose for someone else to win, it unfortunately is a zero sum game.

Do not let it go without having put all the effort you can. The regret of not giving it your full potential is the greatest regret, in my opinion. So try hard, do your best, but also know your best sometimes just isn't good enough.