Try some FFA deathmatch and sweat the DPS support style. Also try 1v1 customs doing similar. Supports should be confident in duelling enemies, sometimes a switch might be necessary depending on who/what is focusing you but you should be able to fight and kill attackers (or even go out attacking on your own)…
Pretty much as it says here. If you need something done then absolutely do it yourself.
This is very good advice for OW matches.