in tears right now and really, really need help with puppy separation anxiety

firstly, I fucked up and didn't do enough work with separation when I got my standard poodle puppy at 8 weeks. i work from home, etc, excuses excuses, and i feel terrible and stupid for not starting earlier. i often left him with sitters and at daycare, but not enough time solo.

puppy (now 7 months old) cannot go more than 10 seconds of me out the door without whining and howling. it's hard for him to have me out of his sight, even when i'm just with another room with the door closed.

i am raising this puppy (my first dog) solo and i really, really need help and advice. today has been terribly hard with standard normal puppy stuff, and the separation anxiety is killing me. i feel like the worst dog owner.

please, any help or game plan suggestions would be greatly appreciated. thank you so much.

edit -- everyone's responses have been so compassionate and kind. i can't thank you all enough.