First of all, I am so, so proud of you for trying to stop. I know it can be very difficult!
The list of distractions that Rising Above linked has some great ideas, and I would definitely look through it. Everything on pages 2 and 3 of that link can be great for soothing stress and anxiety as well as giving you something else to do.
It sounds to me like one reason for cutting is when you look at the cuts, you remember that you were able to focus on yourself while you were making them. So I would recommend that when you make time for yourself, do things that also leave physical evidence. You could try painting your nails, or maybe drawing on yourself instead of cutting. You could also try something like writing about your thoughts in a journal, making some kind of art, or organizing your room. And when you look at those things you can also be reminded of time you spent on yourself.
You also said that sometimes you feel worthless or useless. I've heard that some people will make a big list of all the reasons they are not worthless, and keep it to look at during those times. I've also heard people will simply make a list of good things that happened to them and look at that. If you have a good friend or family member who will reassure you, talking to them about these feelings might help too. You don't have to disclose your cutting.
If that doesn't work, try an alternative method. Take a cold or hot shower, rub ice cubes on your skin, or snap a rubber band. These things can give you a similar sensation, but are much less harmful. You might also tell yourself, "If cutting is what I "deserve", or if it will make me feel better, I will punish myself by NOT doing it." None of these are long-term solutions, but they may help you get a handle on it until you feel better, and prevent you from cutting.
I hope some of this gives you an idea for an alternative solution. Hopefully you will be able to find healthy alternatives for coping with anxiety and stress, and replace the cutting with those until you don't feel the need to do it anymore.