I picked this subject heading because I believe I got in this position with my partner because he couldn't wait to get into a relationship with me before he was divorced, hence the reference to emotional immaturity.
I think that's putting an obfuscating label on a very clear problem: He lies a lot to get what he wants. He won't stop, even when caught.
I actually don't have an issue with trusting him.
This makes no sense to me.
You can't trust him to be honest about anything. He might have an affair and lie about it. He might rack up huge debt and lie about it. He might have STDs and lie about it.
It's bad enough when you're in an LDR. If you're together and married, you can't escape his lies by hanging up the phone. If he racks up debt, you're on the hook for it. If he's having an affair, you've got to deal with the fallout because you're under the same roof and all your assets are entangled.
I have friends who lie. I keep them at a safe distance. I don't let them into my life or my assets in a way where they could cause damage. I definitely wouldn't go into a partnership with one or co-sign a loan or anything like that.