It's something that he felt was right for him, at the end of the day. I understand that him moving onto someone else hurts, but that's a good thing for you, because if someone you are with is so quick to run to a new person, then what does that say about them as a person?
As far as your insecurities are concerned, your feelings are valid. I've had a lot of times where I've felt I'm not attractive enough, or my body isn't enough, or I'm not experienced enough, but still, somehow, I find people who are attracted to me. You may feel that you're average, but you'll still find people who will say you're the most beautiful person they've ever met!! It's crazy how attraction works and how we feel about different people.
My point is, try and put your energy towards people who will appreciate who you are right now, as the inexperienced sub, rather than as what you can be (comparing yourself to other people your Dom has dated).
You can post on Show and Tell if you feel confident enough, or basically talk to a friend about how they perceive you, how they feel you are as a person, why they like you, what makes them want to be your friend?
It's what people who care about us and keep us in their lives who matter more than people who we want to please.
I hope you find the support you need : ))