First is saddle tilt- level or slightly nose up so you're not sliding forward.
Second- if your butt wants to be more forward, move the saddle forward to be under it.
Third- if reach is too long (unlikely with such a short stem unless the frame is too big) raise handlebars or change bars to smaller pattern.
Forth- allow some time to develop the flexibility & core strength needed to comfortably hold riding position.
Assumes that saddle height is reasonably good.