I don't get the sentence "Oh, like I wasn't dreading tomorrow enough, having to give it back to him
(Rachel enters from her room.)
Rachel: Has anybody seen my engagement ring?
Phoebe: Yeah, it's beautiful.
Rachel: Oh God, oh God, oh God oh God oh God oh God.... (Starts to look under the couch cushions .)
Phoebe: No, look, don't touch that!
Rachel: Oh, like I wasn't dreading tomorrow enough, having to give it back to him... 'Hi Barry! Remember me? I'm the girl in the veil who stomped on your heart in front of your entire family!' Oh God and now I'm gonna have to return the ring, without the ring, which makes it so much harder...
——the episode 3 in season 1 of Friends

In the sentence "Oh, like I wasn't dreading tomorrow enough, having to give it back to him... ",what I don't get is why she discribed her dread in the past progressive tense and wether "having to..."is the object of the verb" dread" or just the adverbial adjunct of cause
Thank you

Question

I don't get the sentence "Oh, like I wasn't dreading  tomorrow enough, having to give it back to him
(Rachel enters from her room.)
Rachel: Has anybody seen my engagement ring?
Phoebe: Yeah, it's beautiful.
Rachel: Oh God, oh God, oh God oh God oh God oh God.... (Starts to look under the couch cushions .)
Phoebe: No, look, don't touch that!
Rachel: Oh, like I wasn't dreading  tomorrow enough, having to give it back to him... 'Hi Barry! Remember me? I'm the girl in the veil who stomped on your heart in front of your entire family!' Oh God and now I'm gonna have to return the ring, without the ring, which makes it so much harder...
——the episode 3 in season 1 of Friends

In the sentence "Oh, like I wasn't dreading  tomorrow enough, having to give it back to him... ",what I don't get is why she discribed her dread in the past progressive tense and wether "having to..."is the object of the verb" dread" or just the adverbial adjunct of cause
Thank you

Answer

She says 'wasn't dreading' in the past progressive to make a difference between yesterday, for example, when she was simply dreading having to give the ring back and today - when she has made it ten times worse by losing the ring in question. 
And yes, you could certainly think of the 'having to...' clause as the object of dreading i.e she was dreading having to give back the ring. Or you could think of 'tomorrow' as the object of dreading, and the clause after the comma as simply being an explanation of why she was dreading tomorrow. Either interpretation makes sense.

Answer

Rachel uses present progressive in her sarcastic comment because she already felt dread before she spoke and now that she can't find the ring she still feels dread, though even more so.

"Having to give it back to him" is a separate clause, I don't think I'd call it a true adverbial adjunct. She's upset and speaking her thoughts out loud so she is speaking in fragments.  Both fragments provide information that is independent of the other and either could stand alone.