How do you respond to a girl who wants to break up with you?

The wording matters more than the word breakup. If she says she thinks she wants to, or that she has been feeling something and wants to talk it through, she is still open and may be testing how you respond. If she says it is over, or gives you a reason that sounds final, she has already done most of the thinking and is informing you. Look also at how much she has written. A short message after a long silence often means she has been carrying this for a while and wants it done. A long, careful message usually means she cares about how it lands, which is worth noticing even if the answer is no.

One more thing to check is whether she is asking you a question at all. Something like what do you think, or can we talk tonight, is an invitation. Something like I'm done is a statement. Replying to the statement as if it were an invitation is one of the most common ways this goes sideways.

Your reply should be shaped by her message and nothing else. Three kinds of response tend to fit. The first is a short acknowledgement that you heard her and that you want to talk properly if she is open to it. It is useful when she has left the door ajar but you need a moment. It should be brief, avoid questions that require an explanation, and leave the choice of time to her. The second is a clear acceptance, if the relationship is over for you too or you can see she is set on it. It names the decision, thanks her for telling you directly, and stops there. The third is a single honest statement of what you want, if you still want to continue. It says that once, with a reason, and then gives her room to answer. You can tell a draft is the right kind when it could be read calmly by someone who has already decided, and when it asks her for nothing she is not ready to give. Leave out reasons she has not asked for, leave out a list of grievances, and leave out any line that tries to make her feel guilty.

Send your reply within a few hours if you can, because a long delay reads as either not caring or not knowing what to say. Daylight usually works better than late at night, when both of you are tired and more likely to escalate. After you reply, give it time. If she wants to talk, she will usually say so within a day or two. If she says nothing, that is an answer, and a second message asking if she got your text is pressure. One follow-up is fair if you have something real to add, such as a time you could talk. Two messages without a reply means stop. If she does reply and the conversation turns into bargaining, let her finish her point, then say what you can accept and end the thread there. Her decision is the one that counts, and staying calm and brief is what will make it easier for you to look back on this well.