“I keep trying to lose weight, but it keeps finding me” is a popular saying that has been printed on many gift items, such as T-shirts, posters and bumper stickers. A version of the joke (as a conversation between “First Fatty” and “Second Fatty”) has been cited in print since at least 1973.
   
 
12 July 1973, Charleston (WV)

, “The Gazetteer” by James Dent, pg. 1B, col. 5:
HERE’S a clutch of goodies from the latest issue of Advertising Incorporated’s “AD-Ventures:”
(...)
First Fatty: “I thought you were trying to lose weight.”
Second Fatty: “I am—but it keeps finding me.”
     
Google Books
Building Standards
International Conference of Building Officials
Volume 42
1973
Pg. 32 and Pg. 98:
I keep trying to lose weight, but it keeps finding me. —Norman C. Langen
 
Google News Archive
26 June 1980, Bryan (OH) Times, pg. 6, col. 5:
Nine Day Diet Wrap-Up
By Sharon Patten, Women’s Editor
“I keep trying to lose weight, but it keeps finding me!”
 
10 August 1986, Lexington (KY) Herald-Leader, “Bumper stickers have gotten too smart for their own good,” pg. B1:
I Keep Trying To Lose Weight But It Keeps Finding Me.
   
10 January 1989, The Plain Dealer (Cleveland, OH), “People’s Pharmacy: Diet’s nothing but a state of mind, just think thin” by Joe Graedon, pg. 6B, col. 4:
Not long ago, a friend gave me a refrigerator magnet that proclaimed cutely, “I keep trying to lose weight, but it keeps findin me.”
 
Google Books
The Ultimate Diet Log:
A unique food and exercise diary that fits any weight-loss plan
By Suzanne Schlosberg and Cynthia Sass
Boston, MA: Houghton Mifflin Harcourt
2009
Pg. 141:
“I keep trying to lose weight ... but it keeps finding me!” -ANONYMOUS