How do you feel comfortable with your body around the likes of your partner, when you are not comfortable with it yourself? Why should you not feel insecure about your body with another person; when you are showing another what you do not like?
If you are unhappy with your body weight/shape, are you doing anything about it to try to change how you feel? Are you doing the likes of skipping meals/taking supplements to try to lose/gain weight? Doing all of this because of the belief that your partner does not like how you look is not the greatest mindset, because your partner is with you because they love who you are. Poor lifestyle choices may have led to this, there may have been barriers for example, such as long hours at work, not leaving enough time for yourself to properly live (lack of taking part in physical activity etc) or it may be psychological problems (feeling like you are not good enough etc).
If you may try to lose or to gain weight, then you may wish to speak to your partner about how you feel. However, in some cases, your partner may be reluctant to speak about this in the worry of hurting your feelings – so if you still feel uncomfortable with your weight then perhaps diet but only to a respectful size so that you can find a better and healthier lifestyle for yourself.
Only you can make a difference; this may be through getting a brutally honest answer from a loved one, or by doing something like joining a gym to help you.
Only you can make a change to yourself which you will be happy with.
Find the determination.