The most crucial key to any relationship is that mutual feeling you hold between you both: that you matter. Sounds like you are stuck in a cycle of hearing your ex say you don't matter. That's why it didn't work with him btw. He wasn't reflecting to you that you mattered. However it ended, clearly though that's the sentiment that's lingering with you. So here you are hanging around a new man why is telling you the same message. Move on. You aren't unworthy, you just haven't found a man who is worthy of you!
To be worthy of you, he must see your worth. Often though before anyone else can see your worth, you have to believe it.
We humans are social beings. We learn how to BE in relationship as children. The caregiving you received (or didn't receive) set the stage for how you show up in all your adult relationships. Insecurity in relationships often has much to do with feeling unheard or unseen, perhaps feeling like you don't matter. There are many ways to rewire these relational patterns, the first step of which is taking pause and noticing that you are feeling insecure - so congrats on that because clearly you are already there! Next I'd suggest finding a relationship therapist to help you sort through your insecurities, either as a couple or individually.
I suggest seeking the support of an AASECT certified sex therapist to help work through much of the issues you address, you simply may need some really qualified support. Also, you might be interested in watching Esther Perel's ted talk on the secret to desire in a long term relationship, and/or you might want to sign up for my own 8-part-series on Reconnecting Parent Couples
What Makes Love Last? how to build trust and avoid betrayal by John Gottman is a great book for you both to start reading. You may also want to invest some time and energy in couples therapy. It sounds like there is a lack of trust in your relationship, likely a hangover from your beginnings, that would be helpful for you both to work through to truly be able to forgive and move forward.
Would you have the same desire to meet this friend if she were a he? If you are concerned that their friendship is a substitute for you when you aren't present, talk to him. It sounds like you are concerned that he is building an emotional intimacy with her and spending time doing the sort of things with her that you'd otherwise be doing with him. Help him understand why this bothers you but try to also be open to allowing him to make friends. Is the issue that he hasn't introduced you? Is there another reason you don't trust their friendship?
Sounds like your panic is less about loosing the relationship you are in (which by the way doesn't sound like it's meeting YOUR need for connection) and more about being alone. Being with him is serving you, even if the relationship is less than ideal, from being alone. Being alone can be scary. You'd have to face yourself. You'd have to learn to love and care for and cherish yourself. All important skills and tasks in maturing and growing up, but scary none the less. What if...What if you gave yourself permission to be in a relationship with YOU?
Overly simplified, but that's the trick. When you can let in and celebrate the good stuff everything will start to find more balance. Our brains are fine tuned to focus on and highlight all the icky ugly stuff and file it away to try and avoid it in the future. We don't process the good stuff in the same way. It's left over from our ancestral days when we needed to survive the saber tooth tiger but it doesn't always work so well these days. Slowing down and being mindful of the little things, celebrating the tiniest details of what works...that's how you counter act that stuff. Check out my connectfulness.com/blog you may find some more tidbits there that help you shift gears.
Let's just start with acknowledging that trust is huge and betrayal hurts. You're entitled to your feelings; all of them and you need to know that your husband understands you. That said some ways are more effective at rebuilding and repairing relationships that others. I am a big fan of The Gottman Method for couples therapy, especially following infedienity. You can read about this approach in Gottman's books: "The Science of Trust" and "Making Love Last" and/or you can seek a Gottman Certified couples therapist here: https://www.gottman.com/private-therapy/
The issue at hand here is that you're betrayal broke his trust. In order to repair your relationship you will both need to confront the infidelity. And both of you will need to honor yourselves by communicating your feelings and ensuring that those feelings are heard and validated. It would be really helpful to do this work with a Gottman or Emotionally Focused Couples Therapist. Gottman's The Science of Trust and What Makes Love Last would both be helpful reference books to guide you along.