The issues are largely being driven by the huge demand for housing in Bristol that has seen prices sky rocket. Newport prices are currently rising due to an influx of Bristol residents looking for bargains. Same problem around Chepstow and other satellite towns.
Bristol is a booming city with a big demand for graduates. With the loss of the Pre-Beeching lines and tramways, it makes more sense to find somewhere where you can commute in from outside on the trains than to fight your way through the local roads.
Only answer I can think of is developing the site at STJ and an increase in the services/capacity during commuting times, building a bigger car park, building a new junction for the station on the M48 and having shuttle busses from the station to Chepstow etc.