It's quite common especially with women for them to dislike their body, to see themselves as too fat or too thin, to have the wrong colour hair or curves in the wrong place. We don't stand a chance with the "Barbie" and glamour images that we are bombarded with in the media. It is good to admit how you feel about your body, because it is possible to get such a distorted view of your body that you can get into eating disorders.
What research has shown is that disliking our body or seeing it more negatively than others do is a sign of poor self-image. Your real fear seems to be that if you marry your man, that he will get bored with you and leave you. I'm sure that he loves you for more than your looks. Physical attraction alone won't sustain a long term relationship.
Perhaps it's time to accept that you are attractive and to listen to what others say of you. Remind your self of what others think when you hear the whisper inside that you aren't attractive.
Why not start building on some of the other factors in your relationship other than appearance. Why not consider doing a
marriage preparation course. There are a number of them described here on the site. These will help you to realise that there is more to your relationship than whether you are more attractive than the next girl to walk past your man!