Hi Cat and welcome to the forums
Congratulations on deciding to embark on a healthy eating/exercise program and for losing over 5kgs already, that's amazing!
Sometimes when you know that you'll be attending a social situation/party etc, it's a good idea to try and eat your regular healthy meal first so you're full, and then just have a little bit of food at the party. Don't try and restrict yourself too much, or you'll just end up on bingeing! Maybe see if they have any healthy foods you can have there, like vegetables and dips.
You shouldn't worry about what people think about you either. If you want to lose weight, then do it! Don't listen to those who say you can't or shouldn't - if they are that unsupportive of you, they probably aren't great friends in the first place. Like you said, your health is at risk, so you do what you have to do to rectify that! What would you rather have? A lousy friend who doesn't support you or your health? An easy choice to make I think!
All the best!