unfortunately I forgot my keystore password
This is a problem that has come up many times. If you have lost the password, you cannot use the keystore. You can, however, use brute force apps on the password to crack it. But that may take a while. IF oyu have a "standard" password you used/use a lot, try using this/these passwords and see if that is the right one.
So, without the password you cannot do anything - not enable google play app signing, and you cannot sign it yourself.
And a tip for the future: Write down the password, or use one you know you will remember. The password doesn't have to be very complex, personally I don't even understand why there is a password. But when you have lost the password you have two options (after trying different possibilities): Brute force it or create a new keystore and upload your app as a new one.
Without the keystore password, you cannot use Google Play App signing or sign it yourself. SO you need to get a hold of that password
Or you can see if it is in a temporary gradle file at this location:
..Project\.gradle\2.4\taskArtifacts\taskArtifacts.bin
One brute force option
SO question on the topic
one SO question on losing the keystore password
another SO question on losing the keystore password