Paddy
You have a problem in that you are not going to get a much smaller sail that will rig and work properly on a 460 mast.
Unfortunately longer masts are stiffer (stiffness is rated by the ICMS number). A 460 mast is usually ICMS 25, a 430 mast is usually ICMS 21. What this means is that if you could rig your current 6.0 sail on a 430 mast instead of a 460, the softer mast would allow it to bend more in gusts, releasing the excess power and making it less likely to rip out of your hands. It would give you a wider wind range on your existing sail, especially with your light weight.
If you try to rig an even smaller sail than a 6.0 on your 460 mast, even if it has an adjustable head and the sail physically fits, it won't perform properly and will feel either on or off, throwing you over the front in gusts and retard your learning.
I don't know what year Infinity you have, but if you check the Ezzy website the current 6.0 Infinity (2004) should preferably be rigged on a 430 mast with a 30 centimetre extension on the bottom of the mast. It works on a 460 - but a 430 will probably work better. Check your model on the web for rigging recommendations.
You will find many modern sails are designed to rig on a 430 mast with an adjustable base. This is why there are plenty of 2nd hand 460's in shops for sale, but good 430's are much harder to find.
If you need a smaller sail (and you will eventually), I suggest you trade your 460 mast in on a 430 now and buy an adjustable mast base with AT LEAST 30cms of adjustablity, preferably more (Arrows makes a 40cm, Neil Pryde makes a 48cm ext). Then, you can hunt for a smaller sail in the knowledge it will fit, rig and work properly on your new 430 mast.
For next size down I suggest you look around for about a 5.0 sail, which is getting close to the smallest sail that will work properly on a 430 mast, and I suspect probably the smallest possible sail that will work on a 135lt board. If you try to sail a tiny sail on such a big board in high winds, it will bounce around everywhere in the chop and throw you off. That's why you need a smaller board for strong winds.
Sorry for the bad news, but it's part of the learning curve. Those fanatical windsurfers you see on the beach don't have a van full of boards and gear for nothing - the sail and board needs to be fairly closely matched to work properly, so if you want to sail in all conditions you will eventually need more than one board.
While on the issue, are you applying enough downhaul to the sail ? In strong wind pull the downhaul down as tight as you can so the whole top of the sail goes floppy down to about the 3rd or 4th batten - this will reduce the power of the sail as the top twists off.
Before you buy another sail, I suggest you trade down to a 430 mast, take the removable cams out of your 6.0 Infinity to depower it, heave on the down and outhaul heaps to depower it more and try again. If you're still overpowered (and you will be in anything over 25knts), then buy another sail.
Good Luck !
