My girlfriend got asked on a date...

My girlfriend, whom I have been dating for three months, recently got asked out by a guy she knows on Facebook. She told me about it and asked if I had a problem with her meeting him. I thought she was joking, but she is serious and says it’s a harmless date. She says I am being too possessive and that this has nothing to do with our relationship. It makes me feel as if I am the villain here. I don’t want to be possessive or controlling but all my friends say this is strange behaviour. They think she is trying to manipulate me into questioning myself. I don’t know what to do. I thought things were great between us, and this has made me wonder if our relationship has a future. What should I tell her?
You should tell your girlfriend that gaslighting is an acknowledged form of psychological manipulation, where one person causes another to question their own judgement. This is about control, and she is the one doing what she is accusing you of. Your friends are right, and so are you, because this isn’t what normal relationships are about. There’s nothing wrong with a partner meeting someone, of course, but a date makes no sense. It is disrespectful and serves no purpose, unless she can point out that her relationship with you isn’t working. You’re not comfortable with it, and that is all that matters. If she can’t see that and accept it, you have no future with this person. Treat this as a red flag and an early warning sign that what you both have will probably not last very long.

I’m not happy when I’m single, which makes me worry that I won’t be able to cope if I am not with someone all the time. Is this a problem?
The need for companionship is normal because we’re human. Being unable to manage by yourself is a problem though, because it signifies a lack of coping mechanisms. Speak to a counsellor.