Hey Jenn,
I am so sorry about your situation, but I'm glad you found us here. Most of us have experienced one degree or another of the madness dished out by alcoholic/addicted parents, and so you are (for what it's worth) not alone, and in good company. Lots of smart and compassionate folks here.
Your mother is making choices for herself. Just as you wouldn't want her making your choices for you, you're also not allowed to make her choices for her, no matter how sick and sad hers are.
She needs to get help with her addiction and she isn't (yet). Perhaps your dad leaving is the event that will finally make her hit rock bottom and get some professional help with her addiction and her mental problems. I can only pray that's so.
But it cannot be your cross to bear. You did what you thought was right -- you loaned her money because you were convinced she needed it. You didn't yet know the full truth. Your family events were disrupted through no doing of yours -- you have a right to be angry. Let yourself off the hook -- YOU DIDN'T DO ANYTHING WRONG. You were just lied to, and based your actions on lies.
You can't save her. And you can't save your father -- these are both consenting adults who are creating their own situations. It's a shame that they are trying to drag you into their drama, but you must look out for yourself here. If this is causing your life to fly off-kilter, causing you stress, guilt, physical discomfort, insomnia, any of that ---- you have done the right thing by detaching. Be as protective of yourself as you would be of a small child, or even of your mom in better days. You are the only one who can protect your peace of mind.
Many of us have been forced, against our wishes, to go "No Contact" with our mad loved ones, just to save our own sanity. As they say in the Friends and Family forum, you can "let go or you can be dragged."
Speaking of them, I wonder if you would also be interested in cross-posting your message on the Friends & Family of SUbstance Abusers forum. There is quite a huge and active community over there and so it might be a source of additional support and ideas for soothing your soul. Many a time they have helped me to find out where that line in the sand called "enough" was -- that point where I felt that I had done all I could, and could detach from all of the Things I Can't Solve. They can also help you toe that line between helping her further (perhaps by printing off some resources and leaving them for her) and letting her find her own way.
We are not superwomen. We can only do our best -- and you should be proud of yourself for trying so hard to bring peace to your parent's marriage.
Now it's time to take care of You.
We're all with you.