Extremely confused and need advice on interest

I had my college credits paid for via a scholarship but my gramma was taking out the loans for housing on my account, which honestly I thought it was on her account and not mine. I checked my credit score and saw I owed over $30k in loans, I had no idea what it was, so I reached out and they said it was student loans. So, I called the government loan lady a few months ago and she said I don’t owe anything monthly based on my income, but I should pay around $100 a month to avoid interest from accumulating since I didn’t have any interest. So that’s what I’ve been doing. Today I check my account and I have about $700 owed in interest???? I’ve paid about $1000 total (my gramma gave me $600 like a year ago to put down on it) and my total balance is $31,305. And apparently, just paying $100 a month isn’t enough on my own, I needed to enroll in the payment plan for it to actually count towards anything. Unfortunately this wasn’t told to me months ago when I was making the payments. So that’s $700 down the drain on $1000 paid. I enrolled today in the payment plan that’s about $100 a month. It says it has potential to be forgiven after 25 years. So should I try to pay off the interest owed (the $700) in one lump sum? Or should I just leave it?