How do you let a new relationship breathe while still keeping away the resentment?
This is a good question, that can also be stated "How can I turn my jealousy to compersion?"
In your case, in Hanna's case, she has so little time with Issac as it is. They work opposite shifts! I am not sure they are great candidates for polyamory with their schedule mismatch.
Generally it is recommended that when one's partner is out with another partner, you keep yourself busy with things you find engrossing. Especially as newbies, this is important. Fun hobbies, going out with platonic friends, working out. However some people can't feel compersion until they get a second partner of their own and have NRE to distract them. A tit for tat thing isn't recommended though, as a person who is jealous when their partner is out with someone they are really into might just go out to find someone, anyone, to be with, but not find real satisfaction, and so get even more hurt and jealous in the process.
(Read the blog of our member pulliman to see how his wife didn't get over her jealousy of his long distance lover (for years!) until she got a lover of her own, and how this is impacting the husband/wife relationship now.)
It is very unfortunate that Hanna chose your husband for a bf when he is hardly ever around! And to top that, she chose to not continue to develop a romantic relationship with you, so that you'd have more time to get closer to her husband! When she hardly ever sees HIM because of their opposite work schedules!
Why is she doing that to herself? 3 relationship potentials (you, her husband and your husband) and she's hardly getting any romance! She's gone into martyr mode, sounds like to me.