If it helps, this is a perfectly natural reaction to a break-up.
Part of you misses the familiarity and feeling of security that a relatrionship brings, no batter how badly it ended, or for what reasons, and the urge is to lapse back into familar territory, rather than launching out into the big cold strange world of another relationship.
You must not mistake this rosy nostalgia for genuine feelings - or the willingness to focus on the good and gloss over the bad in your relationship - it is natural, but's false.
You can;t really decide how you feel about your ex if you are still being 'friends'. You are not being friends, you are scratching at the scabbed wounds that broke you up. You may be able to be friends in the future, but you can;t go from one relationship as lovers to another as friends seemlessly, however nice and neat that would be.
Take a complete break from your ex. Don't see him, call him, text him. Give your new relationship a chance. After a few weeks, you will have a better perspective on how you really feel about things. Do you really miss him, or is the loss of the fdamiliar that makes you feel you want him back? You can only assess that from a distance.
Be honest with your new boyfriend - you want to take it nice and steady because you are vulnerable and a bit lost. He will understandm, if not, then he is not the guy for you - which doesn;t mean that your ex is - one does not lead to the other, it's this 'wishful' thinking you must avoid at all costs.
Good luck, take some time, sort out your feelings, and then move on, in which ever direction.