It sounds like you're trying to raise the mast from the front, and then get the side stays on whilst it's held only by the forestay. I'd recommend doing it the other way: get the stays on first and raise from the back of the boat.

As has already been suggested, the trick to doing this singled handed is to tie a rope to the forestay, run it through the bridle and back to the deck. Having put the mast up with both stays on, keeping it upright is just a matter of leaning on it whilst you take up the slack and tie off the rope. You can then go and sort out the forestay.

Paul