To start you need to know how magick works in order to cast a spell. If you can charge/ground energy and focus, then you can cast. Otherwise it's a waste of time.
Next, the threefold law isn't something everyone believes in, so if you don't believe in it you don't have to worry. It's a Wiccan belief, so if you don't follow Wicca you shouldn't consider the Threefold Law. However, if you do believe in it and/or follow Wicca, you should know it's been interpreted a number of ways, even if you think you've lost it all, there's always something more you could loose. Some feel 'threefold' doesn't mean 'three bad things' but 'mind, body and spirit' so by that belief this curse could affect you by mind=anxiety body=sickness and spirit=losing your spiritual connection. If you believe it will effect you physically 3 times, you could crash your car, house burn down, loose your job, be robbed, close friend/family dies or becomes sick, you could become sick or injured. These are extreme examples but I'm trying to point out there's always something worse that could happen. If you're alive, you could loose something. You can also gain anything. [And I know how hard it is to look on the bright side, but try looking at some good things] plus, if you believe in the Threefold, you should also believe it will effect them as well. If you do good you will be rewarded three times over. [It's not just for punishments you know, but rest assured those who did wrong will get what they deserve]
Whether you curse her or not try and forgive and move on. Right now forgiveness seems impossible, but just work on finding inner peace so you can stop the pain and move past it. This anger is only hurting yourself at this point. Meditation, journaling, perhaps therapy, can help. Even yoga or affirmations have been known to help.
I am sorry to hear this happened to you, and I hope you get better soon. [But as a personal rule I don't post curses]