Has anyone had a terrible taper run 3 days before a race? How do you mentally overcome?
The taper is a time when you should feel that 'things are not right' Feeling antsy, hyper and somewhat flat is nothing to be worried about. You cannot make fitness gains in the last week before a race, only tire yourself.
And, in my experience of over 20+ years of racing, is that sometimes you train perfectly and nothing goes right and sometimes you just fly by the seat of your pants and you nail it. There are no guarantees! (but training properly certainly enhances your 'luck'. Don't fret; plan your race and race your plan!
The best way to prepare mentally is to immerse yourself into your most successful past accomplishments. Target those training days and/or races where you had the most success. Try to relive the moment again. Put yourself in the action, as if you are there again (rather than an onlooker). What were the sounds? The smells? What did the ground feel like beneath your feet? What were your thoughts? How was your heart beating? Doing this will help you put yourself in the same mental environment you were in on that successful day. Spend at least 15 minutes each day (more often if possible) visualizing these moments. Nothing is more powerful than your past experiences. Use your successful ones to your advantage.
All the best in your race!