[TBox]

Halfway through this sentence I lose track of which particles fill roles for which verbs:
もう星さんの事はどうでも良くなってて、この関係にず ぶずぶはまっている僕にこいつの…

A native tells me the first part is, "Actually, I've become less interested in Shou-san, but I find myself getting more involved in this relationship," then sort of wandered away (it's the internet...)

And then we hit boku ni. I don't know how much of the preceding is a relative clause, I don't know what verb that "ni" belongs to, and the rest of the sentence is meaningless to me. The bit about her back is because the narrator is embracing a woman from behind.

Can someone help me line up all the parts of this and translate it?