What a sad story. It feels like she's tied your hands.
Since she said don't mention getting back together, you need to respect her request. However, you need to understand that every action, in this action, her decision to end the relationship and not consider resuming it, is motivated by all kinds of thoughts and feelings. When the feelings and thoughts are thoroughly discussed, a person has a chance to resolve them, and, therefore, doesn't need to take action. She didn't talk to you, so she took action instead.
This means that there is a chance that if we get her talking about how she felt about you and the relationship, and especially get her talking about her negative feelings, the talking will resolve the feelings and she may become willing to try again.
The key is to encourage her to talk about the feelings that prompted her command that you not ask about getting back together while not making her feel that you are violated her demand and asking her to try again.
Here's the tricky part: When to talk to her, she needs to feel that you are respecting her edict and not pressuring her to change her mind. Instead, she needs to feel that you are just trying to the goal of understand her and her feelings.
To achieve this, you might say:'I want you to know that I intend to honor your wishes. I won't pressure you to get back together with me. What I would like is to understand the thoughts and feelings that led you to end our relationship. And what made you not want to get back together.
Since I love you, it's important to me to understand what's going on inside you. Tell her also that you have the feeling that she had a lot of negative feelings toward you that she didn't say; and that they eventually built up to the point that she wanted out. You would like her to tell you those negative feelings that she was too polite to say before.
Have her tell you what did and didn't do right with her. Listen, repeat back what she says to show you've heard her, and, above all, don't defend or justify yourself. Also explore whether she didn't want to try again with you because she's afraid that you'll let her down again.
So, in other words, is her wall a protection against being hurt by you in the future. Whatever she tells you, thank her for her honesty and tell her you'd like to hear more. If you do this, you will be doing everything in your power to get back together. There's a very good chance that several discussions of this kind will do the trick.
Wait for her to say she wants to try again. If it turns out that there is too much water under the bridge, then I'm sorry for you. At the very least, the conversations will provide you with feedback that will help you be a better partner next time around.