Repair it stronger than it started.
The trick is not simply stronger thread. It is spreading the load across healthy fabric with a backing patch and dense, overlapping stitches.
Never sew only through the torn edge. That edge is already weak. Build a new load-bearing zone around it.
Use these materials
Thread
18/3 linen thread for very sturdy hand repairs.
Alternative: Gütermann linen No. 30.
Needle
Sharp hand needle with a large enough eye for the thread.
Bohin big-eye sharps are a good upgrade from generic repair-kit needles.
Patch
Similar or heavier linen, washed first.
Aim for ½–1 inch of healthy fabric beyond the damaged zone.
Why your repair keeps breaking
A single tight seam follows the original tear. The force stays concentrated in the same tiny strip of fabric.
A patch behind the fabric plus several stitch rows creates a larger load-bearing area.
Best hand stitch
Each stitch overlaps the previous stitch’s territory, creating a stronger continuous seam than a simple running stitch.
Do it in this order
Do not enlarge the tear.
Extend well into undamaged fabric.
Small stitches, relaxed tension.
Use two or more parallel rows for crotch, seat, or knees.
Too much thread tension can make the thread cut through the linen.
The “stronger than original” recipe
Washed linen patch + heavy linen thread + backstitch + multiple rows + relaxed thread tension.
For a recurring failure spot, reinforce the area before it tears next time. Think of the patch as a structural plate rather than a bandage.