How to get insurance when im 7months pregnant?

There is no such insurance for foreigners who don't reside in Germany. The available ones all either don't cover pregnancy and childbirth or have a 9 months waiting time.
Your only chance is marrying your boyfriend and getting a family reunion visa - then you can (and must) join a German health insurance, which covers all medically necessary treatments without waiting time or exclusions.
Since buerocratic processes are slow, it is very unlikely that you manage this in the two months until you give birth. Thus it is likely that you and your boyfriend will have to pay for the birth from your own pockets. If there are no complications, this will cost €5000-10000.
Note: If the child is German citizen (due to the father being German), you can after birth get a residence permit due to being his/her mother and caregiver - and then again you and child can (and must) join a German health insurance.

Flying during an advanced pregnancy is more risky but not unheard of. There is no law as far as I can tell, just policies which differ from airline to airline. Most ask for a medical certificate for woman beyond the 36th week. This is not necessarily a problem to get at 7 months if one is young, health and has had no complications – and chooses to take the risk. If an airline has claimed that it is not legal - then they have misinformed you. Maybe you just need to check out different airlines?

Here is a link to a site showing the policies for 25 of the most major airlines for pregnant women flying:

https://www.tripsavvy.com/flying-while- … es-3994876