If you want my honest opinion, none of this sounds like you're "scared to be alone". It very much sounds like you're worried what he might do, hurt about all the secrecy around it, and know in your gut that you can't ever be 100% sure he won't betray you again.
The only part where "maybe I'm just scared to be alone" seems to be kicking in is where you desperately try to convince yourself that you do trust him and he would never do anything, and therefore need to find a different explanation for your feelings.
You commented on swinging from "I love you" to "I hate you" rapidly. Is that new / specific to this situation, or are you normally like that?
It always worries me when people are having to work so hard to find alternative explanations for how they are feeling besides the blindingly obvious, most sensible and logical one.
I have this horrible feeling in my chest I’ve been carrying around.
Is that feeling loneliness or is it not? Is it more impending doom/betrayal? Is it the same feeling you had after finding out he'd cheated?
Our bodies can communicate things to us that our minds aren't ready to accept yet.