What is a serious relationship?

Im almost 20 and have been in a relationshio for a year and a half. I was so sure i loved him but now i dont know any more. He was by my side as i was getting stable from being being bipolar and stayed when i cheated 3 times. I completely am better in every way but he upsets me so much now so im unsure if i should stay with him. He always has something negative to say either my hair looks a mess or im dressed too casual ( before it was too dressy) i need to work out and get healthy and dont walk on that side of the side walk. He always wants to have sex and i got so tired i cried once. I felt violated. He keeps me as a secret from his parents. I dont want sex every day. Im asking for advice and answers to what really is a serious relationship.