You're right about issues with the 3200 being mainly mental. In many ways, it's the toughest race in high school track. And yes, the third 800 is the toughest part of the race.
kid gloves is also right. Start by focusing on your first 400 to 600. If you've been struggling, there's a good chance you're going out too fast. It's not a middle distance race. Let others take it out and jockey for position. Hold yourself back a little. As you move from the first 800 into the second, begin to gradually move up to where you want to be competitively.
The idea is to leave yourself a little stronger for the third 800, which is the key to the race.
Once you've passed the 1600, this is where the mental challenge is the greatest. You simply can't run this part of the race thinking about how much is left. That's guaranteed failure. As you put it yourself, take a leap of faith. Go for it. It won't all work perfectly the first time, but this approach is the key to success.
Good luck!