You sound a bit like me, my shoulders are as strong as my chest so I do find it hard to isolate.

Using dumbells helps. And Heavy dumbell flyes is good as rather than pressing you are moving in an arch as is you are hugging a tree so the shoulders are removed a bit from the movement.

Other ways are weighted wide grip dips.

Or, you can pre-exhaust your chest with dumbell flyes first and then do benchpress afterwards.

Paul.

Note: warm up well if doing flyes first. They can put a massive stretch on your chest, so you need to be careful with heavy weights.