Has the future already happened according to special relativity? – NO.
In fact, in special relativity, the question of whether or not an event has “already happened” depends on the observer and has no meaning if the observer is not specified.
I find it so hard to believe!! – THEN DON’T.
Believe this instead (but only after making sure that you understand it):
What is true according to special relativity is that for any distant observer relative to whom you are moving sufficiently rapidly, some events in your future may be seen as in their past relative to the time on their clock at which you think they are now (or rather at which you will think they were now when you eventually see that “now” event in their lives).[And for every event in your future there are some possible observers in your “now” (though you will not have actually seen them yet) who, when they finally see that event, will judge it to have happened in their past relative to the time on their clock at which you (will) think they are now.]
So in the world of special relativity, there is no time-ordering of events that all observers will agree on.