Shower gets colder during use, already replaced cartridge, what else could it be?
Depending on the type of valves you have, it may be that the washers on the hot side are expanding as the warm water heats them, closing the valve slightly, and the reverse is happening on the cold side.  If that's the case, and you don't want to live with it, then the only solution I can think of is to replace the type of valve to one that is explicitly designed to avoid this behavior.

If it happens more towards the end of the shower, rather than the beginning, then it may be that you're running out of hot water.  The easy solution there is to turn up the temperature on the hot water tank so that you use less of it during your shower, and space out activities that use a lot of hot water.