Your happiness is clouded by your past experience (with your ex) If you have not fully resolved the pain from the past infidelity, it is influencing you in your current relationship.
You would want to separate both situations (what your husband did and your fear that your new partner may hurt you). The truth is, whether or not you fear that he will stop loving you or hurt you is not something you are in control of (his behavior) It helps when you remain grounded in the present and direct your energy onto making the current time enjoyable.
It sounds like you've become quite attached to him and thrive on his attention. It is natural especially when you feel strong feelings towards him. Yet, even the best relationships are defined by boundaries (yours and his) You do not want to become enmeshed (that is not always indicative of love but rather of dependence)
Emotions are dependent on our thoughts (what we think, ruminate over and what we accept as our truth) It may be helpful to start paying attention to your mentation and how it results in certain feelings such as fear/anxiety, love/dependence, etc. Then you can consciously redirect/reshape these thoughts to where they are healthier for you and the relationship as a whole.