Finished six months chemotherapy. Trying to get back in shape by physical activity. When I just don't feel good should I push it or just rest?

Rest. You just finished six months of chemotherapy. Let your body rest. Listen to your body. Do what make you feel good. If working out too hard is making you feel "bad", back off, relax. Try again the next day...good luck. Hopefully everything is a success with the chemotherapy.
Congratulations. on completing your therapy and being in the process of getting back in shape. Check w/your doc to make sure your workout plan is ok for your health status. Then stay balanced, i.e., see if you can do half your plan on days when the thought of exercise seems like too much. Also, mentally working out can have benefit so do some guided imagery, especially on the half days. Peace and good health.