I think it sounds like you're FA. Remember that the label is to help you figure out what your roadmap of things you need to heal and work through is to increase your own security and cope better with stress and be happier in life... it is not to say FA is "worse" than any other attachment style. It is a little more complicated to heal than AP, because you've got dual competing fears to address on both the anxious and avoidant sides of the spectrum, but it's easier to do when you're aware of it and know which issue / attachment style you're trying to address.
Therapy and the help you've already sought out on your own is a good start. You can also look through anne12 's healing tips for FA for a lot of information. Another thing to remember is it is possible to reach secure attachment within a specific relationship (like with the ex you felt secure with) without having overall changed your insecure attachment style. So while your ex was a safe person for you, there was more independent work you needed to do to heal and shift your entire attachment style, but you weren't aware of that yet which is okay. But that's why when you met your FWB, who is not an emotionally safe person for you, you ended up back in a chaotic anxious-avoidant trap situationship. The FWB is triggering for your existing issues in a way the person who made you feel secure, though possibly bored, was not.
It doesn't matter why he "stays" and we don't know why, but I'm inclined to say it's because your issues trigger him as well so he is drawn to the toxic dynamic, plus he's getting something out of it from you without you requiring commitment... so then he "stays" because you allow it and always reopen the door for him to be FWB without commitment, which is what he's comfortable with. It's the pattern for these types of painful pairings until someone decides they've finally had enough.