The "forget" option tells the Android system you no longer have the card. What it then "forgets" is the encryption key needed to encrypt/decrypt this specific card. So basically you've locked the safe and threw away the keys; without the keys, there's no reasonable way to open the safe again.
Thus, your data are lost. It might be possible to regain the key (e.g. if the device is rooted; you could create an image of all its file systems and use forensic methods to find and extract the deleted key). But not only would that require your device to be rooted, the effort to find and extract the key is not low, and chances are it's meanwhile overwritten. If your data are worth a 4-digit value (EUR or USD) to you, you could contact a forensics company to give it a try – but chances are rather low.
Permit me a note to one of your comments on your question:
I do not have any backups, because all worked properly.
That's exactly the point-in-time when one does perform a backup: you need the data intact. You cannot create good backups of a broken system – but at that point you'll need a backup to repair it, as we've just learned.
Also see: How to get Adoptable storage encryption key without root access after I “forget it” (answer: you can't).
