Dear Lom 1979
I think the problem you have, actually is because the opacity texture of your material has plenty of dark parts at the edges. All you need to do is to increase the color intensity for the edges (make them more white). Here I have reproduced a very simple material that I hope can help you.
As you can see, like you I have a torn cloth with dark colors at the edges which cause the loss of material in those areas. The first multiply node only shows the base red channel texture (which is used as opacity mask), the second multiply is set to significantly increase the color intensity to white (in order to fix the material loss at the edges), by doing so, some holes on your texture will also become more white which may cause them to lose their transparency. To fix that I have subtracted a small number which makes those areas black again (in order to keep them transparent). You can repeat the process of multiplying and subtracting for several times to get a better result. The next multiply actually is used to refine the result and reduce the transparency of the cloth itself. At the end you can use a power node (if you want), which makes dark areas darker (it actually makes tiny holes in your material to show up more clear).
Note that the numbers that I used may not exactly bring the result you want to achieve, so play with the numbers to get the best result for your specific material. One more thing to keep in mind is that Opacity, determines translucency while opacity mask determines transparency. For opacity mask, engine reads white color areas as opaque and the whole range from pure black to white as fully transparent. For opacity, engine reads all the values from 0 to 1 as translucent (0, fully transparent. 1, fully opaque and the whole range between, as translucent)
But to get the best result, you need to modify your masks inside Photoshop or something.
Regards,