Socialising makes it hard!

I understand that. And, to be honest, became a little anti-social to battle it.
Here's something I've realized: If you complain about people "sabotaging" your diet, people are going to stomp all over it and tell you to take accountability to for you own diet, blah, blah, blah. But I find calling it sabotage provides me what I need to say no. It makes me a little angry. The harder I'm pushed, the harder I push back. For example, I get mad at coworkers for bringing in donuts. I don't let them see that I'm mad, I mostly just grumble to myself. But that little bit of anger keeps me away from the donuts.

But, you're probably nicer than I am, so here are some other suggestions:
- Before meeting with friends or family, tell them you're on a diet and don't want to eat anything. If there are specific foods you're not eating, tell them what foods. I.e. "Mom, I look forward to seeing you tomorrow. But, I'm not eating sweets anymore. Will you please hide them from me?" People want to be supportive, but I think most don't know how.

- Use the rule of three. Make it a habit to always say no 3 times. Even if you're thinking "I'm gonna have this anyway" say no 3 times before you give in. Three is an arbitrary number, but it shows up in a common theme in fantasy literature: Say someone's name 3 times and they appear, say something 3 times and it can't be a lie, demand something 3 times and you will be obeyed. It's fiction of course, but I find basing my arbitrary rules on fiction to be effective for me.

- Throw it away. I find if I can't resist the candy dish, the best trick is to pick out a piece and throw it in the trash. Or I'll take a cookie off the plate, and throw it away. For social purposes, you don't want other people to see you do this. But psychologically, it's very effective.

That's all I've got right now. I'll check back in if I come up with others. Ultimately, you'll have to try some different things and do what works for you. Good luck!

Your friends are not responsible for you, you are responsible to hold yourself accountable.

One thing that can be very useful is to plan social events around activities, instead of food and alcohol. Dancing, skiing, biking, hiking, etc.