There might be supplementary insurance that can help. The Medicare part D plan should be able to help. If you are not eligible for any coverage that could help there is the Patient Assistance Program where a medication company can provide the medication without cost. Medicare should be able to help since you said you have it. Try calling their hotline. If they are unable to explain things try the Medicare Rights Center. If a medication is working you shouldnt have to change it.
I am on medicare Part D and take Abilify. Without Part D it is $600, with Part D it is approx. $200. Then, of course when you hit the donut hole after$2500 has been spent on meds the cost goes up to half the actual cost - $300 a month. Part D is not very good at reducing the cost of meds until you have spent more than $4550 in a year. If your income is low enough to get medicaid as your supplement then your med cost goes down below $10. Check with your county social services about getting help. I am always amazed that people think Part D is a cureall for getting your meds. My monthly cost at this time is $500 a month on Part D.
Bristol-Myers Squibb has a patient assistance program for Abilify. They say to call them if you have Medicare part D. There are some discount cards out there that are free to download and print that may give you more of a discount. Tell your doctor you're having issues paying, and ask if they can give you more samples while you try to work things out. I'm not sure about Medicare, but with other insurances, if your doctor tells them that nothing else works, they can cover it when they normally wouldn't.
Please talk to your doctor now to get started with your options, before you run out and end up stopping cold turkey.