when I had the same problem, I tried to use square knot at the end of the string (use a needle of a wire to push the knot as close as possible to the beads) then, put a little dot of super glue then pull / hide the knot in one of the bead. Hope it can help.
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Super Glue and Stretch String by: Anonymous
I found hard glues like super glue caused the string to break faster. It appears that by making that spot inflexible the next spot has to take all the movement.
Also crimp bead edges cut into the cord right away and all the way through as the stretch cord is moved.
So far the only way I've found is to have clamps to hold the cord ends to hold the knot and put fabric glue or costume crystal glue.
It's a pain and you have to wait for the glue to dry but seems to last much longer.