How to reply when she mentions seeing someone else

She told you about another guy for a reason. This isn't accidental. Either she's giving you a chance to step up, she's testing whether you'll get insecure, or she's honestly torn and wants to see how you handle it. The fact she's texting you still means something.

She's also filtering for neediness. A lot of guys either beg for exclusivity or pretend they don't care. She wants to see if you're grounded. Someone who can acknowledge what's happening and still be unfazed. That's different from being indifferent. You can want her and still not spiral when she mentions someone else.

Move one. Acknowledge the situation without making it weird or asking her to explain. Don't ignore it, don't ask questions about him, don't act surprised. She already knows what she said.

Move two. Redirect to what matters between you two. A good reply shows you're grounded enough that this doesn't shake you, and it keeps the thread going in a direction that works for you. Short replies work here because they don't overshare or overexplain. Long replies look like you're thinking too hard about it.

Reply within an hour or two. Waiting days looks like you're hurt. Replying too fast looks reactive. A normal gap shows you saw it and you're not spiraling.

After you send it, step back. Don't text her again until she responds. This is the hard part. If she doesn't reply, or if she keeps dating the other guy, that's information. It means he's winning or she's not that interested. Don't double text, don't ask for clarification, don't check in. Just move your attention elsewhere. If she comes back, you'll know she was thinking about you.