Definitely look into IBR/ICR repayment options for your public loans! You can go right to
http://studentloans.gov and look up information on the loans you have, and all the different repayment options, and you can fill in deferment & adjustment applications right online.
Don't worry whether you've filed your taxes yet or not. I put off applying, because I hadn't filed, and ended up in default. Sallie Mae did offer me a reduced repayment then, which got me out of default & fixed my credit, but the amount was still
way higher than my income-based rate, and more than I could have possibly afforded if we were paying for housing. When I finally looked at the IBR application, it turned out that all I needed to do was tick the box for alternate proof of income, and send in a couple of pay stubs. I could have saved myself about $4000 if I'd just applied right away!
I don't know what to tell you about the private loans, as I never had any, but it might be worth looking into some kind of refinancing or consolidation loan. In any case, I find it a bit dubious that you'd *have* to go into default first. I would recommend you do some thorough googling before you do that, to see what other options there are, and what (if any) regulations there are on what they have to do.
At the end of the day, I don't
think it would come up in your citizenship. I don't think they do routine credit checks, and I don't recall them asking about it on the form. I think the only way it would come up is if it got to the point where there was a court judgement, but the finance companies generally try to avoid that. They'd be more likely to sell the debt on to another company first, and all that can take quite a while. So, even in that worst-case-scenario, you could probably apply and be finished with your naturalisation process before it ever came up.