It's an almost-quote of an old blues song called "Going Down Slow". I know Howlin' Wolf recorded it, but I'm not sure if his was the original version of the song.
The relevant verse goes:
Yes I've had my fun
If I don't get well no more
Yes I've had my fun, baby
If I don't get well no more
You see I have this pain in me
And I'm going down slow
Which, of course, ties in with the vibe of "I Want You"
I think he's saying that that in being corrupted or compromised, she's now indebted to him - that she must recognize this/him as invested as he is in her. The lines before it say more or less how much he wants her, the lines after it underscore this sense of indebtedness to him to the point where he affords to taunt her bitterly and is crazily obsessive, so it could be seen as a pivotal line.
I agree it may be a drug or alcohol reference. Long-time junkies craving a fix don't refer to needing to get high - they say they need to get well. Boozehounds sometimes say a similar thing, particularly when hung over, hair of the dog and all that.
The song may be saying you've had enough fun, as in enough getting high or loaded. There'll be no more "getting well" for you.
"But it's a dangerous game that comedy plays
Sometimes it tells you the truth
Sometimes it delays it"