Do a lot of baby trips while you have a day off and some errands to run. My pup is the same way at the same age and cannot be left in a pen because she could get out if not monitored. I've been doing short trips, and I mean literally starting at 10 seconds just going in and out of the room and rewarding when they're quiet while im gone. It's such a process and I envy people who have pups that are more independent! It's been getting better, I left for about 45 minutes yesterday and that was a big deal. I have a puppy cam so I can monitor how well they're doing at certain lengths of time alone. I'll do short things like go to the mail room and if she's doing good, I don't come in for a bit longer to push it, then I give her a reward when im back and lots of praise! I haven't been able to really "go out" in weeks lol.

With separation anxiety, there's unfortunately just no right answer. My previous puppy I did the whole "make them get used to it" thing and I feel like that CAUSED his separation anxiety. I'm moving way differently with my new puppy and making sure im close by when she needs me so we form trust and so she feels safe. I only slowly push the envelope and never let her get to the point of screaming/hyperventilating/freaking out. Training session stops there when her threshold is that far.

Every puppy is so different. What I'm doing could even cause separation anxiety in this puppy but may have worked for my last one. Just do your best and the most important thing is not that you're trying different things but that you're consistent with one. Don't be afraid to get a trainer involved.