Please get her spayed, then re-evaluate this. There really is no point in all this worry and hand-ringing until you've at least done that, otherwise you are just wasting time, money, and effort. 8 months is certainly old enough to be spayed, in fact most owners do it long before this.
If the problem is that she is "marking" the couch, then this problem will likely to away once she is spayed. If the problem is that she really does feel that it is the best place in the house to relieve herself, then you need to try other things: try different types of litter, add more boxes, get larger boxes, get uncovered boxes if yours are covered. If you are worried about her peeing on a new couch after you replace the old one, then you might want to try things like making a point of restricting her access to the room with the couch for awhile until you are convinced her litterbox use is rock-solid.
_________________ The little monsters 3
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