I'm new to poly stuff so I can't offer solid advice. But I'm sorry to hear that you are struggling in that relationship and that you aren't getting the security you need from him.
From some of the other threads I've read it seems that taking a break from other relationships may not help, especially if he's not onboard with the idea.
It may help if you try to address the personal issues directly. It sounds like you needed to know he was still there for you even though he as with someone else. I totally get that.
If he decides that he wants to work things out in your relationship, maybe set up some expectations like what's considered an emergency, how can I contact you if it's an emergency, etc. And then maybe you could think of support people you can call or visit when he's with other partners and you are feeling worried but it's not an emergency.