Update: I fixed it and i'm now getting text messages from iPhone users.
I did a few things before finding out it worked, so i don't know exactly what did it. Here's the order in which i did them.
1) Logged in to apple.com and deleted my iPhone in my list of apple products (manage account, find the list of products/devices you have)
2) logged on to iCloud and deleted my device (my old iPhone)
3) went to the self de-registration page (Deregister and Turn Off iMessage - Apple Support) IN SAFARI instead of Chrome and it now correctly sent me a deactivation code. However, once i entered the code, it said that number was not registered as an iMessage number, so the solution may have been 1 & 2 above. However, if you're having trouble getting a code sent correctly, try using Safari b/c, you know, apple.