Hi /fit/, I'm a fatty. I gained about 20kgs in about 8

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Hi /fit/,

I'm a fatty. I gained about 20kgs in about 8 months from a combination of depression, lack of discipline and inactivity. I'm 20 years old, 5'9 and about 92kg. My question is, my birthday is coming up in October, and I don't want to be a fatty for what could be one of the more memorable moments in my life. My question is, how much weight can I realistically lose in about 4 months? I can stick to a training regime without too much trouble( I used to cycle competitively, training 15-20 hours a week), but I struggle with diet. What do I need to do?

Pic related, it's me when I was fit.