The first benchmark to fix in your mind is how much money does the US Government get from all sources, other than borrowing, each year. It's about $2.4 trillion ($2,400,000,000,000).
The third benchmark is the amount that the US spends on entitlement programs (social security, medicare, medicaid, government and veterans pensions mainly). Entitlements cost about 1.8 trillion.
Interest on the US debt is about a quarter trillion ($250,000,000,000).
Uncle Sam gives almost all of the money he gets to old folks, veterans, and people who need medical care. What's left, about half a trillion, isn't enough to do everything else. Hell, defense alone costs more than that. So that's why we need to borrow half a trillion a year.
The reason it's not sustainable is those entitlements and the interest on the debt are growing fast.
Non-discretionary Costs Drive Overspending
It's crunch time unless we roll back those expenses or jack up taxes. If we jack up taxes, who's going to pay them?
Who Pays for the Federal Government
Well, take a look at your W2 form. See where you fit in the table above. It's you and me who are going to pay, and our kids, and our grandkids. Oh the rich folks will pick up about 2/3's of the tab, just like they have been doing, maybe more if the income tax gets more progressive. But all of us will be paying something, and it will hurt.