iMessage Activation Error

I recently changed my Apple ID from ***@Gmail.com, to ***@icloud.com. I have changed nothing else, yet when I tried to sign into iMessage, it would load for a bit, then come up with a popup saying, "Activation unsuccessful, please try again."

I've tried MANY different solutions, none of which have worked, and my friend has tried signing on on my computer, and it worked perfectly. Please help :/

Also, I've been to heaps of other topics on websites about this same problem. DO NOT RECOMEND THE APPLE SUPPORT PAGE ON THIS TOPIC. If you can't sign in to your iMessage account or FaceTime on your Mac - Apple Support is completely useless and didn't help at ALL, and every post about this issue always has this as the "Best awnser."

Another website I've been on is http://www.hawkdive.com/2017/03/imessage-activation-error-occured-mac.html, which had about fifteen different solutions, none of which have worked. I have tried all of them.

I'ld also like to point out that iMessage is working on my phone perfectly, and this is only happening on my computer after changing my Apple ID. Please help me.

Also thank you in advance.