It is a huge disappointment, I am in the exact same boat when I got a BFP first attempt I was thrilled. But like you hcG didn't double and I miscarried. Very heartbreaking.
I think if you don't have full confidence in your FS then wait. I understand this comment is a touchy one but: because you are younger you really do have the luxury of a little more time. That time is well spent to be more confident and happy with your FS.
Another option might be to change clinics if that is available in your area.
One thing I am thinking about after reading your egg numbers is first attempt was 5 right? Did the new FS bump up your drugs to get the 12 eggs? Reason being, my FS says less eggs of good quality are better than lots of eggs that are poorer quality. Perhaps the frozen embryos are not of the optimum quality? Some clinics will freeze embyros that other clinics consider unviable. It might be worth making sure your clinic is using best practice in that way- cause nobody wants to waste time and money with a clinic who are a bit dodgy.
Also, did you have pre testing done eg of the sperm quality? Because you have POCS it might be worth linking in with a thread on that for ideas too. One thing I remember suggesting to a friend is Clexane- it can help implantation as it thins your blood- she is now pregnant after 3 years if IVF. Maybe a different drug regime might work better? The other thing to look into is suppliments- low zinc can cause failed implantation too.
I am only quite new to IVF and I am sure others with more experience will weigh in with some great ideas for you.
I know it must be hard not to let it get you down.