You need a plumber to sort your pipes out, to determine if you have any old galvanized somewhere out there. Replace if you find any.

Replace the heater. The cold supply side has to be 3/4" as mentioned, with a new shut off valve. The hot has to be replaced as I described in my previous posting. Run a copper line from the T&P valve of the new heater to a drain near by.

Not sure? get the plumber to do it.

Once you do all these, there is a good chance your pressure will be good.