threesomes?

I'm so fucking scared and confused. What does it mean if your man asks you if you'd like to be in a threesome with him... we have been dating for a year and now I feel like I don't fulfill him enough so he has to bring in another girl... I really truly thought this was my soulmate because everything has been going so good but we have recently just been having some troubles with sex... I just don't think I'm good enough for him what so ever. Ladies, have any of you been in this kind of situation and how did you handle it?... 😭😭😭