I would suggest "moved on" or "was pulled a different direction"
... especially if it was a pull factor that changed the focus...but not only so.
"I'm not doing X but I'm doing Y" is far more positive that "I'm not doing X anymore" . It emphasizes that the person is still confident and striving, just in a different direction.
"I went as far as I could." is another good way of saying more that you finished after giving a full and valiant effort.
To fit your example sentence, instead of:
I recognize this former goal and I let go of it."
I would suggest:
I moved on after going as far as I could with that past goal.
I moved on to new goals.
I went as far as I could with that goal.
(the issue with "write-off" is that it sounds like the person believes that the energy put in was "misspent" ... which isn't a matter of "not being gentle" but a matter of saying the wrong thing.
While "let it go" might be somewhat better, it is still suggesting that there was a futile obsession at one point rather than ending with plainly a rational and happy completion and succession to other worthy interests)