Ryan Buckman, 26 years old, was previously 22 stone before he decided to create his own diet and exercise regime.
The Essex schoolteacher gave up alcohol and stopped going for romantic meals in a bid to shed the weight.
He then celebrated reaching his target by completing a half-marathon, just before he was given the elbow by his girlfriend.
However, Buckman has gone on to better things as he has now landed his dream job as a PE teacher, as well as starting a relationship with fellow teacher Kimberley.
He said: "Looking back I did get obsessed with the calorie counter; I was on it all day and wouldn't eat anymore when I reached the maximum.
"But my ex said she didn't like the attention I got when I lost weight. It pushed us apart and it was sad at the time and I ended up losing too much weight.
"But now after meeting Kim my life feels perfect. She helped me put the calorie counting into perspective and I gained a few pounds to look healthier."
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Buckman had always aspired to be a PE teacher but was too embarrassed of his weight to apply.
He commented: "I'm so proud to be able to help the kids at school who saw me fat and know it's possible to get slim and healthy."