I really sympathise with you, we meet a lot of clients with the same issue and we know how distressing it can be.
I strongly recommend you try the Nioxin system; it cleanses, revitalises and stimulates blood flow to the scalp to help promote fuller thicker hair. As for hair extensions, with the root of your hair being in a weakened condition my fear would be the weight from the added hair would make matters worse.
I know it's frustrating but the health of the hair and scalp is priority. In the meantime Bumble and Bumble do an amazing thickening range, try the full form mousse and the dryspun to create a full, airy silhouette.
I hope this helps!