Happy to report I completed my Rescue Diver certification this past weekend and am now a Rescue Diver (not to be confused with someone who can actually RESCUE someone, but I think at a minimum I could help try to save someone if necessary)
Don't underestimate yourself mate. You done the course and are already more competent in "rescue" than the vast majority of divers. Rescue is not something black & white that some people can do, and some can't. That little thing you do may just be enough to save someone. That experienced rescue diver may just be too late to helps someone in a case that seemed 'easy'. As you say, the least thing you can do is try to help, and improve the chances of someone recovering or surviving.
Just done the course myself and during the course I too thought "I'm never gonna save someone", but I know that every little bit of help may just make a big difference.