I'm in the Nether and can't get to the Source Portal anymore. I built my 2nd portal in the upper world (I destroyed the first one) and went through it. Now on the return journey but always a new portal on the coordinates of the 1st created where I then come out. Can I do something, or do I have to accept it?
The coordinates (x / z) of a portal in the Nether must always be the coordinates of the corresponding Overworld portal divided by 8.
Then you have no more problems.
Go to the portal in your Overworld. Divide the coordinates by 8. Go to the Nether. Build a portal at the calculated coordinates. Now both portals are perfectly connected.
You do that with all other portals, too, if you have more.