Please can someone help me, I am very confused indeed. :-/

I had a small combi boiler fitted a few years ago, but since then my hot water pressure is usless and my cold is really fierce. It wasn't like this before.

It was too hard to regulate a mixer tap shower so I bought a triton cromo single lever mixer shower which we had plumbed to the mains pipes, not the boiler.

But again we are having issues with the fact that too much cold and not enough hot. When turned down to the right temprature it dribbles out of the shower with not enough power to shower a gnat.

And now I have been told that as I have a combi boiler I can't have a pump as it has its own. Is this true? Even if the shower is not fitted to the boiler itself?

If this is true how on earth can I sort out this water pressure issue.

Please, please someone help me.