My water gets cold very quickly doing dishes or showering

Define what "quickly" actually is... such as how long.

Gas or electric heater? Back to you, Tom

There are several possibilities which can cause this problem. Find a good plumber or possibly spend hundreds replacing parts until you happen to find the problem by randomly replacing parts, unless you have the skill to do some testing and analyzing on your own.

Has it been like this for a long time or just start. Could be mant things like a bad dip tube, failed stat, low voltage

Before we can give solution we have to know what we're dealing with. Gas or electric Back to you, Tom