Why can't I fight this off?

Try to going to a different doctor if you decide to go to a doctor again. Sometimes it's easy for you to get sick right after being sick because your immune system is still weak so you're vulnerable to illness right after fighting off a illness. I've also been getting sick on and off but I eat poorly and dont excercise I also think it may be an allergy to my dog. Maybe you're allergic to something. I wish you the best of luck and I hope you get better and kick that cold!