That's a tough spot you're in. If you are sure you can't afford to take on the risk, stay firm but be flexible on other options.

Explain to her that you are looking at what is best for your family, and just don't feel the extra debt is a good thing right now. Tell her you're willing to work out some other way acceptable to both of you to get her what she wants. Make her come up with another plan and then support her with it. Better if she chooses it than you, since she wants to be independent. Best wishes.

M: A really long time.
Crisis: 5 years.
She's still worth it.

Life is never made unbearable by circumstances, but only by lack of meaning and purpose. -Viktor Frankl