Cutting glass with a diamond is something requiring a lot of skill and familiarity with the cutter. I could cut glass with my own diamond but not with my fathers. There was a subtle variance in the angle to hold it. Much easier to use a wheel type cutter where the world and his dog had every chance of success. Forget clamping and creating stress in the glass. Lay it on a flat clean sheet of wood with a cloth cover such as baize. Use a straight edge to guide the cutter to score a line from edge to edge of the worksheet. Carefully insert a bit of dowel under the score when the score will crack through and the two pieces separate. Old glass is a lot more difficult due to brittleness. Don't press too hard, there's a correct pressure which you can learn with practice on scrap pieces.