Should I reply to my ex-girlfriend's text message? ...

It's not a lover who can miss it. Although maybe she was really waiting for your reply, you didn't reply, so she was disheartened, completely desperate and found a new boyfriend. But this seems to be a regret and cannot be changed. Since it can't be changed, leave it alone. It's not a problem to reply to text messages now. You will regard it as a problem because you haven't put it down yet. Actually, you can return it or not. It's no big deal.