I would like to support the side of you that exercise and live life rather than feel sorry, or obsess about how you look. So maybe you are tiny bit heavier than some average. You should not aspire to be average. Maybe you are a little smarter than average too, do you feel compelled to act dumb to comform? Some do, believing it is unfeminine to be brainy, or to be proficient in technical fields or math, or play certain sports. You will hear lots of b.s. on the topic of how girls should be. Just ignore. Why on Earth should you try to fulfill people's stupid ideas of how you should be instead of being yourself?

And it is natural to have low times, most people have bad moments. Your daily cycles, monthly cycles, even amount of sunlight out there all affect your mood subtly. When your mood is low it is easier for negative thoughts to float up. I really admire you for not surrendering, but battling them. Please don't fall into a trap of getting a hasty label, such as bipolar, stuck on yourself along with pills tht will cure lows, and dampen your fighting spirit, replacing both with zombie-like blah.

Unless it interferes with your life. Unless you can't function.

There is a lot of self-help available to help you combat the body-image and negative self-talk. Maybe you could even benefit from a good talk therapy, a skilled therapist will help you shine light on internal workings of these thought forms and help diffuse them.