Should I have my dog put to sleep?

This question is so hard for me to even ask. My baby Sadie is 7 years old but we have only had her for 9 months.The past few months she has gotten sicker and sicker. We have taken her to the vet and they say she could have several differant things but they can't tell for sure unless they do more test and we just don't have the money. It's 2:30 a.m. and I am up with her because she was vomiting. She vomits several times a day now and never really moves. We have to hand feed her and give her water or she ends up vomiting more. She has lost so much weight that you can easily see her bones. She can't eat or drink without throwing up some of it. We have tried giving her high calorie foods to gain weight but nothing works if she can't keep it down. I know that the right thing would be to end her suffering but I don't know if I can let her go. I'm crying right now just writing this. My mom offered to take her to be put down I don't think I would be strong enough to do it. What do you think?