As far as I understand, both "attempt" and "try" describe active measures towards achieving a goal. Words like "aim", "seek" are listed as synonyms, but seem not to indicate active measures.
What is the difference between:
I am trying to fix the car
I tried to fix the car
and
I am attempting to fix the car (sounds unnatural)
I attempted to fix the car
Personally, I think that "attempt" indicates one shot quick look (attempted, but was too much effort) and "try" indicates prolonged efforts (done everything I could)
Best Answer
To my ear (product declaration: British-born wrinkly), your theory is right, there is something pessimistic about "attempting". There is no attempting, Yoda should have said, only the doing or not doing. When Eliot, perhaps echoing the Gita, wrote, "For us there is only the trying; the rest is not our business", to my ear that suggests a sustained moral endeavour.
I agree with you also that "I am attempting to fix the car" sounds a bit off, perhaps because, as above, it suggests that you don't think much of your chances.
Other NS may differ in their perception of the nuances, though.