I am not sure about your rigging but I assume (from memory) that you have a fractional rig, with aft-swept spreaders, no backstay but two fixed "runners" or normal runners.
First thing to check is if your mast is centered. Take the main halyard
to the toerail and adjust it so that it just touches a reference point (stanchion). Now take it to the other side and compare. There's a little difference when your halyard
is on a masthead sheave that isn't centered at the masthead.
Next thing is the tension on the capshrouds, which also act as backstay. For your rig, they must be tensioned to 20% of their breaking strength. You do that by measuring their stretch on a 2-meter section. I explained that in detail in another thread on this forum, use the search function to find it.
(a fractional rig needs 15% of breaking strength tension but if you have aft-swept spreaders, regardless of fractional or not, you need 20%).
Your baby stay attaches to where the lower spreader is? I find that strange because you wouldn't need it in that case (the aft swept spreader pushes the mast forward instead) so I guess maybe you don't have spreaders where the baby stay attaches. But the fact that a strand broke there, plus the lack of tension on the leeward shrouds under sail point to too little capshroud tension.
You need the runners when you fly a spinnaker/genoa only. If you have the fixed runners you are fine. Tension the runners for mast bend and fore-stay sway.
If you have intermediates: you pretension those to tight while looking up the mast-track so that the mast is straight. Final tensioning is done under sail by tensioning the leeward one to tight while counting the number of turns. Tack the boat and do the same number of turns on the other side.
The lowers can be trouble. These are probably the same size wire as the capshrouds and should also be tensioned to 20% of their breaking strength. If you tension them, you change the tension on the capshrouds too, plus you might not be able to tension them enough. The trick is to loosen the capshrouds again and tension the lowers more and re-tension the capshrouds again and check everything and repeat if needed. You can use a runner to help you tensioning the capshrouds.
The forestay isn't tuned for tension but for mast-rake only. You tension it with the capshrouds and runners.
there's a good little book from one Mr. Dedekam (?). I mention it in the other thread also, incl. ISBN number.