Tips on cutting binder’s board better

I’ve recently started my bookbinding journey. I’m trying to cut the 2.5mm binder’s board using an Olfa SAC1 knife that many book binders use. But I find that I have to cut over and over again until it goes all the way through the board. And then it is uneven, slightly jagged, rather than clean and straight.

Any recommendations for a more heavy duty knife?

I’m using a cutting mat, 90 degree square and ruler already for measuring and square angles.