I presume your ticket is something like
this -to Victoria specifically, not London Terminals.
Three Bridges appears to be the appropriate routeing point for Hassocks for this ticket.
In which case, you can go to Three Bridges, then onwards on the mapped routes on maps:
LB
LB+SL
SC
If you download the
Maps and go to these pages, you can use any route you can trace on a single map (or combination of maps for LB+SL, changing maps exactly once at a station named on both LB and SL) without going through the same station twice.
As you have a season ticket, valid for unlimited journeys, you can switch between any mapped route at any intermediate station. (I'm not sure if the train has to stop at such a station)
LB includes a route via London Bridge, but wouldn't allow you to go to Charing Cross, Cannon Street or any station north of Blackfriars, as there is no way to do a complete journey Hassocks - Victoria via these stations without passing through the same station twice.
You can also use any route not longer than 3 miles more than the shortest route that regular passenger services can take (with no doubling back through the same station twice), calculated using distances in the National Rail Timetable, which is a somewhat laborious process to calculate.