Should I have said nothing and not said goodbye
No you should have stopped talking to him the minute his messages became inappropriate and disrespectful to not only his girlfriend but you. Do you not think that someone in a relationship is being disrespectful to you by thinking that you're open to being the chick on the side? If you don't, then you should probably work on your personal boundaries and sense of self-respect.
He has a partner and has had for the past 4 years so I knew it was wrong to text him but continued to anyway
Yes, definitely work on your personal boundaries so that you don't lose yourself and your good sense for superficial and fleeting bursts of excitement. You've allowed yourself to become addicted to his BULLS*it.
He's not a good person. Just how great do you think he is as a life partner if he was carrying on with you like he was when he has a girlfriend. If you accept what a lousy LIFEpartner he would actually make then you will be able to stop thinking about him. If you accept that you made a mistake, forgive yourself for that mistake of not looking after your own best interests and learn the lesson that your interaction with him was meant to teach you, then not only will you be able to forget about the piece of crap, you will go forward that much more savvy and you won't let a man in a relationship talk to you for more then anything innocent and platonic.
You'll be fine. Start to forget him by immediately changing the subject of him when he pops into your head instead of sitting there in your reverie swimming in your thoughts and memories... I know that's what you're doing now and its the worse thing to do if you're trying to forget a mistake in your life.
Good luck.
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BTW: You don't need him to give you "closure." You gave yourself the well needed closure when you told him that holding your breath wasn't going to cut it for you ~ which it shouldn't. Stop looking for an excuse to contact him and to even give him another thought. HE.HAS.A.GIRLFRIEND.OF.4.YEARS. That is enough closure for any chick with a good sense of self-worth to stop the thoughts of him and what-ifs.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion