Ruining it would be giving up at this stage and your not doing that. Like the others said, it was just a blip - everyone has them. Put it down to experience and move onwards and upwards. Try thinking of ways to avoid reaching for the tuna in future i.e. drinking btl of water straight after swimming can make you feel full, time it so yr due a shake then, perhaps avoiding the kitchen area etc ...
Its a new day, just start again - good luck