Dear Younger Barb,
Keep your diary. You'll be a writer eventually and you'll regret throwing it away.
Your father is wrong about the Beatles, but he is right about almost everything else.
You can do math despite your grades. You just find it boring.
Even the worst experience and the meanest people can be writing material.
Travel as much as you can while you still have your health.
Take more photos of your family and friends. Keep printed photos. Don't put important memories exclusively onto technology that will disappear.
Keep paper copies of all your writing. One day floppy disks will be unusable.
You'll go on a lot of blind dates. Most will be okay, a few good, one horrid-but-hilarious (good material!), and you'll hit the jackpot with one.
The blind date experience is also good practice for writing query letters. Keep submitting.
Keep up your fitness routine. But avoid the high-impact stuff (jogging, skipping, step aerobics) and protect your knees.