internal coolant leak

I have a 1998 Nissan Sentra with 78,000 miles. A few months ago I noticed that my coolant reservoir was empty and I filled it up. Three weeks later it was back to empty. Previous owner told me the car had overheated on them a few months back. I went to my mechanic who told me that my radiator cap was broken. Of course three weeks later I found out it wasnt since I was still losing coolant. There are no signs of external leaking and this time my mechanic told me it was an internal leak and that the coolant is leaking but is being burned off. Also he mentioned that it would not be worth it to fix so I should just fill up the reservoir every few weeks. I bought Stop Leak powder and put half into my radiator without any success. A few weeks after I did that, I noticed that my coolant reservoir became very dirty and contained some oil and I was still losing coolant. Should I put the other half of the Stop Leak into the radiator? Will I be fine continually refilling my coolant reservoir with coolant? What will happen if I continue this pattern?