First of all congrats on your engagement!
I was in a similar position too. I'm 23 and engaged to a tall, fit and handsome guy. We haven't set a date on the wedding yet, but I know how you feel being with someone ad it feels like you don't know why they're with you and what you did to deserve some wonderful guy.
Start small. Really small. It's how I got where I am today. It took me years, but because I changed my habits over a period of years it made the whole weight loss part pretty simple.
Pick something to do first. Exercise or eating right. Do not focus on portions just yet, but pick one of those to do and get used to doing it. I started with exercise, but you might want to start with eating healthier foods. You will FEEL better when you are used to one of these two things in your life and then adding the other will not be so hard.
If you start with eating right, just focus on eating more healthy foods like fruits, vegetables, lean proteins. Learn to cook healthy meals and how to make them tasty so you want to eat them.
If you start with exercise, start small! Go for a walk to a certain point outside and walk back. The next day try and go a little further, and then keep challenging yourself. Make this a regular routine and soon you will crave more physical activity.
Once again
don't try to change everything at once. Start small and you'll be successful. Focus on health first and weight loss second.
Good luck on your journey. We are all here to support you