I ran the numbers on a significantly similar situation I'm in, and here's what I determined.
1. Keep in mind that inflation is working for you. Your loan forgiveness may result in $85,000 of taxes, but that's in 15 years. That's equal to ~$55k in today's dollars (assuming 3% inflation). In the meantime your income increases. So if you are being paid $80k per year now, and your income adjusts only for inflation, you should be making $112k in 15 years. Makes things much more manageable.
2. If you get pushed against the wall, you can always do an installment payment on the taxes you owe to the IRS (and state). You can typically stretch them out over 5 years. You'll pay interest on it, but right now it's 4%. Not ideal, but better than losing sleep.
3. You can purchase real estate now, let the increase in the value over the next 15 years grow, then take an equity loan out to pay off your taxes. You won't pay taxes on the equity loan proceeds. Although you will pay taxes whenever you sell the home. That's probably the best situation, assuming you are comfortable with the risk that the home may decrease in value moving forward.
4. Second best option, and probably a good back up option, is to plan on having enough in a betterment account in 15 years to pay it off.
5. Third, you could continue to contribute to a Roth IRA, then take out the principal (tax free) when you need to pay your taxes.
6. Fourth, and probably the worst of the three but still a possible option, would be to take a loan out on your 401(k) or an early distribution to pay your taxes.