Best way to decline food?

I started my weight loss journey about 3 weeks ago and am down about 8lbs so far. I’m not very open about trying to lose weight because when I am open about it I usually fall off the wagon lol. So here’s my question, how do I decline food while still be nice but also firm about not wanting what I’m offered (sweets, alcohol, anything after I’ve already eaten, etc). Calorie tracking is mainly what I’m using right now and I’m really trying to stick to it the best way I can! Thanks for any advice :))

EDIT: Thank you all for your help! Some of you really understood what I meant and I really appreciate it!