Just a suspicion. If it is getting so that you have less hot water now that the outside air is becoming colder, I suspect the pipes are exposed to the outside air temperature (crawl space under the house, in exterior wood frame wall with no insulation, in attic, etc.) and you are losing most of the heat from the hot water just to heat the pipes up. By the time the water gets to you, it’s only warm.
Is the water heater gas or electric? If gas, there is a temperature adjustment on the front of the water heater down low where the gas line connects. If electric, there will be two covers on the front of the water heater that you will need to remove to adjust the setting, make sure you do not touch any wires when doing so.
Do not adjust the temperature above 140° F as water at 140° F can cause 2nd and 3rd degree burns on adults in less than 5 seconds (on children it does not even take anywhere near that long).
If you have the temperature set at 120° F, run the hot water at the closest faucet until the water coming out of the faucet stabilizes in temperature, is it hot enough? This will let you know the temperature of the water without having to heat up a lot of piping.If you have a thermometer of some type that will measure to at least 160° F you can hold it under the water and check the temperature.
If this is not hot enough, then you may have a water heater problem.