You say that you don't feel like you're good enough for him anymore. What self attack! How about, he may not be good enough for you anymore! I want you to seriously examine your tendency to turn your anger back on yourself. You are furious at him, I'm sure, and instead of saying so, you blame yourself. Misdirected anger is a no no that leads to all kinds of problems from depression to anxiety, diminished self-esteem, and physical ailments.
I want you to work on owning your feelings and directing them where they belong instead of misfiring them back at yourself. As for him, he needs to tell you why he's behaving the way he is. Is he looking for a way to satisfy a natural craving for variety? Is he doing this to cheat, without actually cheating? Is he looking to be kinkier with you and afraid to put you off?
And, here's a deep one, is he trying to satisfying an exhibitionistic urge (a normal phase of childhood development) that was crushed in him. You know, all little boys and girls go through a phase in which they want to flaunt their genitals. This is a vitally important act that the self uses to blossom. If this need is crushed by a parent who is shocked by the behavior, the need goes underground and often reappears later in life in various compulsive behaviors such as the need to expose oneself (flashing). Your guy may very well be trying to relive a phase of his development that didn't go right.
The problem is that he may not even be aware that he was damaged during this phase, and so he won't be aware of the fact that his compulsion to show off his 'pee pee' stems from a wish to make this wound right. As Ricky Ricardo used to say, he's got some 'splaining' to do. He needs to put all his cards on the table and tell you, as best as he can, where he's coming from (no pun intended). He needs to dig deep into his childhood and look for the links between then and now. Has he always had this urge to behave this way? Has he done it before? Does he expose himself in public?
If I am right, he may be working out a problem that has nothing to do with you. In case the behavior is also being triggered by something wrong in the relationship, he needs to ask you what message his behavior is sending you (that he wants out? that he wants you to be kinkier); and he needs to tell you how he expects you to feel about this (does he want to hurt your feelings, does he want you to be unbothered?)
I would also ask him how he feels about flaunting his masturbatory prowess in front of you? Does he feel inhibited doing it with you and safe doing it with strangers? Again, how does this relate to his childhood and the wound he suffered. If he won't talk openly to you, then you might consider going to a couples therapist who can help you navigate the discussions.