How do I get muscle tone and a healthy heart without losing weight?

Patient: I’m a 27 year old female who is 5’10” and weigh between 135 and 140 depending on the day. However, I hardly ever eat because I don’t get hungry and when I force myself to eat because I know I have to I get really super sick. And then when I do get hungry and eat it’s usually fast food burgers and candy and usually late at night. Most of the time it’s the only stuff I can keep down even though I love me some fruits and veggies. I know this is not healthy and I know my heart is not healthy, how can it be when I don’t exercise and eat junk. I would love to be able to walk up the stairs or two blocks to the park without being totally out of breath. However, I’m afraid to exercise for fear of losing weight. Every time I start an exersie plan I drop weight like crazy and look sickly and anorexic. I’ve been to doctors who have done blood work and endoscopes and a whole slew of other tests that all come back normal and then I feel as if they just want to shove pills down my throat for symptoms that they can’t find a cause for. Pills that don’t help and leave me feeling even worse. Heart disease is very common with the women in my family and I know I’m slowly killing myself because of my eating habits and the fact that I’m a smoker (although I have been trying numerous methods over the past few years to try and quit)I just want to be healthy. I want muscle tone and to be able to eat like a regular person, what are some things that I can try to be able to achieve this without becoming all skins and bones from over exercise?