my ex and I have been broken up for 5 months now and I've tried moving on but I can't go past kissing any guy?

I don't know why. But I haven't had sex with anyone since my breakup the most I've done is just make out with other guys. I can't have sex with anyone it seems too soon 😭 and feels weird to realize he really is gone. What should I do when guys ask why I won't move past kissing?