How am I ment to feel good about myself?? !!!!!!!!!!!!!?

how am I ment to feel good about myself when people around me look good without really having to try, im 20 with a child but i look like a 20 with a saggy 40year body, even woman who have had 4 children can get away without wearing a bra! ive been inviting to the beach with college people who are goign to be running around in bkinis, im shitting it I never go to the beach, swimming all everything which shows myself, i feel so awful, if i was 40 it wouldnt be so bad but Im 20!! and my looks have gone completly after one child!! i feel old and ugly...surrounded by normal looking bodies, fk models in mags im compering myself with normal girls and woman, and I look awful compared to them! what the hell am I ment to do i hate the summer i just want to cover up from head to toe! IM HAPPY THEN