There are so many variables, as not all GB masts are the same.
some are made of wood, some of steel, some stand on the upper deck, some on the back cabin top, some have attachment points for a block and tackle, some only for stays, some have already been replaced in order to better manage lowering, so are better designed.
Some detail of yours is required before a lot of posts are wasted in idle speculation.
That said, since you are having trouble, my guess is that you have a steel mast that pivots on the lower deck step, but the mast step provides no side support whatsoever (common for GBs) I have seen this solved, but only at great expense. (on a GB 36 that had a substantial roof structure over the flybridge, which was used to provide support to the lowering mechanism). I don't recommend that approach.
My choice, many years ago, was to lighten the whole affair, so that it would stand on its own (no stays) and could be lowered by one person using a single part line. Mine is made of Aluminum pipe, is supported on a mast step on the lower deck, that has approx 10" tall side supports, so the mast doesn't get to fall sideways on its way down, and in the upright position is supported by a bracket attached to the original base position on the upper deck, this 3 steps up from its new base. This mast is taller than the original, carries all the lights and lines of the original, plus a Radar Dome. The hoisting/lowering is done from above, with a single line to the base of the radar mount, and it has a stand that is pinned to the back side of the mast, that, when lowered, stands it just above walking height over the back deck. I have been quite happy with this arrangement, as I need to raise and lower every time I go out and back in, to my shelter. OK, I admit that is occurring less frequently since I have moved to summers in outside moorage.