Someone help me with hair problems?

my hair is naturally thick, and it is dark brown. i wanted it a shade darker than my natural dark brown, but i didn't want to dye my whole head. so i wanted low lights and the hair dresser said lighter would look better so i just went with that, but i hated it. so then a month later i went to another hair dresser and she fixed it and made the highlights darker, what i wanted and i loved it. now its a little faded out and the highlights are starting to come back out because of the sun. and i'm going to dye it back soon but i'm worried about my hair because i'm starting to feel it get thinner, is there any shampoo i could get to help? and also maybe a shampoo that will also help get the lights out? thanks.