I completely empathise with you. I have been involved with a serial cheater, who is very narcissistic..I have only just gotten to realise this, and see through the wool he pulled over my eyes..He told me he was single, and that he shared a flat with a little 'flatmate' whom he has known for 20 years, and who helped him when his marriage (one of 2) ended. It later came to light, that he was her partner (most would not be so naive to be fooled by this!). he started supplying me with clothes and scent, which I later discovered were her rejects. He even gave me a bagful of knickers, which were tatty and worn, thinking I would be so grateful! These were also hers.... The guy was arrogant as hell, and his seductive charm had me completely captured, until I made myself wake up.
I wondered why he could never spend New Year with me, or even one Christmas evening, or one birthday. This is what in the end made me determine to get rid of him. I could not stand being the spare part, tossed aside for the faithful partner anymore. It was the utter humiliation, not to receive one text or one greeting, and knowing he was spending the most important days of the year with her. Ladies, and gentlemen, please, please, do not settle for such a demeaning relationship. Have self respect and get out there and find someone who will not cheat and lie on you.