You can't work with mycoplasma in your cell cultures. Therefore you must find the cells vial with mycoplasma, and use the cell passage where don't find contamination. If you don't find contmaination in the cell vials maybe you are working in non optimal esterile conditions.
Try to clean severely your working area (laminar flow bench) with antimycoplasma solutions and detergents and use a new medium cultures. Or change the air filters if they are in poor conditions. Also you can ask to lab partners that review your movements. Perhaps you don't realize doing something wrong. As you can see exist a several possible causes. You must find the problem and try to aviod the future contamination problems.
Good luck, and remember to work in strict sterile conditions.