This is pretty rare.
Greg Norman, Phil Mickelson, KJ Choi had all won tournaments with graphite shafts. I suspect that it was the quality of the shaft and perhaps age plays a part as 845 were pretty old clubs and I presume you have not re-shafted them since. Even if they were steel, the rust would have also weakened them by now.
If it were due to being graphite, you would have broken them when they were new, and not now, unless your swing speed have increased exponentially over the last 15 years
If so , then it is quite possible that the torque of the shaft was too high for you. The tensile strength of the particular shaft could not sustain the twist of the head. But low torque graphites are very expensive, much more than steel.
Can I suggest you speak to a certified club fitter and get their opinions too? Then you can make a more informed decision especially since you like the feel of graphite.
I do find KBS steel shaft having the soft feel of graphite so you may also like to consider those.
Somewhere out there is a graphite shaft you can't break unless you tried to. It's a matter of analyzing your swing and impact and figuring out what parameters are optimum for you.
I snapped 3 Aerotech shafts right above the hosel and then became so scared of throwing another clubhead off that I pulled them and moved on to C-Tapers. I don't think that your swingspeed is doing it so much as the down force that your are creating when the club strikes the turf. None of my shaftbreaker swings were anything out of the ordinary but it does happen. I will admit that I now have a phobia of graphite shafts in my irons now! My swingspeed isn't ridiculous but it is pretty high (120 mph) but I know that I am really aggressive down and through impact so that is what I'll attribute the shafts failing to.