I have a HUGE decision to make!

I recently signed a contract to build a house. My son co-signed so that I could qualify for the loan, but said he did not want to help make the payment (he lives with me). I don’t charge him rent b/c he helps me in other ways – like with his special needs little brother. He is a recent college grad and is using his full time job (he’s a civil engineer) to pay off student loans and save for his future. Also I don’t want to depend on him for the payment b/c it will be my house. He will move on at some point.
The payment will increase my monthly housing payment by 50%. I pay $1500 now, new payment will be $2250. I technically could afford this, but my current job is all commission and my sales have been down last couple of months. I am also working on launching my online training program for parents and my speaking business. So lots of variables involved.
My question is this: I am feeling like I’m in over my head and spending money I don’t have. Also I wrote a check for the earnest money and don’t have money to cover it. I thought about canceling the contract and playing it safe. Then I realized I was playing it safe and that was easy/comfortable but maybe too safe.
I also I think I should use my energy to believe I can DO something beyond my comfort zone rather than BUY something beyond my comfort zone. Even if I could make that increased payment I feel I’d rather spend the money on other things. The house I live in now is perfectly fine – although I rent it.
Am I being reasonable or scared?