Okay....deeeeeeep breath!
It can take awhile for all to get used to each other. One thing that will go a long way is if
you calm down.
Here's what finally worked for my hubby. We adopted a sweet little yorkie boy at a year old. He'd had lots of trauma in his young life and it was a challenge in the beginning.
Outlook is everything. All I saw was the wonderful things that were happening. DH was overwhelmed by the negatives...
Finally, one day I calmly explained to him that I would like to give things a try for another 4 weeks. If at the end of that time, dh wasn't feeling better about things, we'd take him back. No questions asked, no grief from me.
When dh stopped seeing everything through the lens of "oh my gosh, what have we done, look at how this has upset our lives" and started seeing things day by day...he fell as in love with this little guy as I was.
Sometimes we get too wrapped up in the big picture. Enjoy the good moments. Buy some belly bands (this will greatly reduce your stress). Realize the bad stuff is temporary (if you have to tell yourself you'll find him a new home in 2 months if you don't feel better, tell yourself that). It's too overwhelming if you let it be.
Find things to enjoy about your new pup. Find quiet moments to spend with your other dog. Realize that the turmoil is temporary.
Then you can get to business making things work...but you've got to relax first
Good luck!!!