you sound like you're not over your ex so you can do one of two things ...
cut all contact with your ex,focus on your new partner and build a new life for yourselves or...
spend some time being single and thinking about what you really want.
it wouldn't be fair to meet up with your ex whilst still being with your current partner and i don't think you should be in contact with both as these situations always turn messy. and no going behind anyone's back as someone will get hurt. good luck, i know its not easy xx