If you have been playing for a decade and still can't control your stroke, then there is clearly a problem. At the very least, you admitted that you have a problem there, so kudos.
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You know what the backhand is supposed to look like right? Have you ever tried to copy the motion in front of a mirror? Seeing what you're doing is very helpful for self diagnosis, assuming you can see what is wrong. A video is even better, because it let's other people see what is wrong.
I think it's about time you give us a video. By the way, when you make the video, please let us see you doing various other shots and points so that we could accurately see what level you are at. Until then, we can't really give you solid advice.
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You talk about powerful big stroke. That is one thing I do have. The only thing that is a problem is how to control all that power. When I do try offensive gameplay I sometimes manage one or two good shots, but they are inconsistent. Sometimes, (and sometimes means almost 80% of the time) they go out. I'm never out of power. But my backhand hits the ball aggressively into the net, or off the table. I can hit a ball standing far from the table and it will still go off table from the other side. It doesn't go on the other player's court.
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You mentioned that your bigshots are never well controlled. Out of curiosity, are you doing fine with driving the ball with the backhand?
If not, then clearly you need forget your pride and relearn the basics. Your 10 years sounds like my 2 years.
If driving the ball is easy for you and backhand looping is a problem, then I have a feeling that you aren't spinning the ball. Either that, or you aren't spinning the ball forward.
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Honestly, your problem sounds like a problem that a tennis player trying to play table tennis would have. I can easily imagine a tennis player trying to make big backhand shots that don't go anywhere on the table.