Separation training help for a clingy 9 month old pup (please I’m losing my mind)

Hi everyone, I’m kind of at my wit’s end trying to work on separation training for my 9 month old pup.

I work from home so he’s used to having me around… and when I’m not around, he usually had my ex’s dog with him.

Now, he doesn’t have another dog around to “depend” on, so we’ve been trying to work on separation training every day.

I’ve done structured training reps about 4-5 days/week and some days are better than others. I start with 30 sec reps and eventually move to 2-3 min. Some days he’s able to get to 5-6 min with no issues, other days, he loses his mind at 30 secs

I know it has a lot to do with how he’s feeling when I start the reps (is he tired? Activated? Etc?)

But I think I need help drilling this idea in his head. I’m super stressed and truly haven’t been able to leave my house for even the smallest errand bc he panics and will start howling

For reference, he’s a mutt about 30 lbs. I believe he’s part dachshund, part chihuahua, maybe even part golden

I’ve had him since he was 8 weeks old; he’s unfortunately not crate trained

Please please any advice or tips would be greatly appreciated 😭😭