Help, iM desperate

Help, I have a 2002 nissan altima. The check engine light came on awhile ago and when I took it to the shop they said I needed a new cadalidic converter. Another place said I did not need that but that I some other sensor, but that it wasnt immediate. Fast forward a year and the car starts acting up and we notice that it is low on oil. We put oil in and now the oil is either burning or leaking out quickly. 2 different mechanics looked at it and saw no leaks, however 1 did say we needed a new converter. We replace the converter and yet we still have the same problem. Now the car wont even start, although I think now it may need a new starter. The problem is that I am still making payments on this car and cannot really afford to continue payments on this car and get a new car. Is this car worth putting any more money into and will I need to get a new engine if so? Also I fear taking it to nissan (or rather towing it there) as it is about 45 miles away and I feel they may take advantage of me. Any advise?