What Causes Tiredness And Difficulty In Concentration?

HEllo Doctor . i am having a problem 2 years ago i had a fever and after that fever was okay but i got glands on my back head under ears i went to doctor he said me that its just becuase of infection nothing to worry but i search the over internet abt those gland and i have read tht due to blood cancer its happen and i was soo worried after that i dont feel hungry sad all the time . my life is hopeless and worst but after some day there is no more gland on back but from little thing like pain or flu came i think i had a serious disease and making me worried all time now currently i feel almost tired all day and i feel like that i m not in present when i talk with someone i cannot concentrate my eyes to focus even i go to gym i feel like no energetic and feel that i m not present like foggy mind what shud i do please help