Itz been almost 6 months that i can't sing well, never hapen like this before. when i sing my voice gets crack and my chest get tired?

Asthma: Please get a "pulmonary function test" this will help us sort out your problem.

Get examined: By your dr. Possibilities include laryngitis , too much phlegm production, vocal cord nodules, bronchitis , etc i hope u don't smoke which can aggravate your condition. I'm from the philippines and with this condition we drink warm ginger/ tea concoction. You may need spirometry to check for airway narrowing, laryngoscopy to check for pathology in your vocal cords etc hope u feel better!