Hello /fit/, I've been fat all my life. at age 18 i was

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Hello /fit/,
I've been fat all my life.
at age 18 i was 175cm tall and was 100kg.
i got diagnosed with a thyroid underfunction, got medication and lost 15 kgs in a span of two months.

now im 21 and im still sitting at 83 KG's .
im going to go to australia in 3 months as a Backpacker, so i want to get down to 75 kg.
the main problem i have with losing weight is that i smoke weed evey day, during the day i dont eat much but when i come home in the evening i smoke a joint and turn into another person and stuff my face with calories. Also drank about 4 Beer a day the last months, but i stopped doing that 2 weeks ago and feel much better already.

i dont want to stop to smoke since its like a medication, but i also dont wanna go to a country where ill spend lots of time at the beach and be a fat fuck.
got any tips for me on how to keep losing weight and keep my discipline? or is stopping to smoke the only way?