I agree that the wisest thing is to tell your mom. You’re right that it wouldn’t be appropriate to swear her to secrecy.
I’m glad you’ve sobered up and left your friend’s house. The big buzz probably wasn’t as great as it promised to be.
You do need the therapy, so you do need the money—and your parents genuinely love and care for you. It is nice to have love in your corner.
Your father is a grown man. My father was a huge overprotective overreactor, too, so I know exactly what you mean. I’m sure if you tell your mom that you are extremely sensitive about the assault and don’t feel comfortable going into detail with your dad that she would know how to manage things and protect you from further emotional turmoil.
You need to weigh pros and cons.
Pros: 1) loyalty and support from loved ones (imperfectly expressed though it may be)
2) the money for therapy
Cons: your dad going off the deep end and the scenes you want to avoid at all costs
Note: If you stay quiet because of your dad, do it for your own sake. Don’t worry that sharing your secret would hurt him. He is a grown man and will eventually come to terms with it.
My own advice: share with mom, get the therapy, bite the bullet, and wait for the storm to pass.
And stay sober. You know drinking is really just a waste of time.