It is not just you, and you have made some very keen
observations in your post. Sailing off the wind with a strong
steady breeze can be as easy as winging out the headsail
and keeping the wind astern. Other times it may feel like it is
impossible to get a feel for. As you noted, your apparent wind
can drop to almost nothing and tell tales quickly become
ineffective below 120deg AWA and mainsail trim is not a obvious when
running in lighter winds, you can pull in or let out several feet of
mainsheet with no discernable effect on performance.
The most helpfull device is a masthead wind indicator, like the
ubiquitous Windex. If you keep your boom close to perpendicular
to your AWA, you will be in the ballpark. If you are not using your
wisker pole on the jib, then you will need reach up to get any
effectiveness from the jib. Moving the lead outboard and forward
will help in this regard.