My only problem is my will power

Aaliyah • Periods are annoying
I have done exercise a few times and it feels very rewarding afterwards. I just can't seem to hold the exercise routine up. I can choose to not eat fatty foods sometimes. I think my only   problem is I see it as rude to not eat ALL the food I get given. Plus it's soooo delicious.  
​How do I develop a better will power? To exercise and diet. Any tips or ideas?