stamina breaking when i let go of shift?

when ever i click shift everything goes fine, but when i let go of shift i want the stamina to regen, and i want to be able to run at anytime as long as the stamina is above 0, but when i let go, i cant run anymore, i know i am doing something wrong.

---sprtsp = sprint speed
---regspd = regular speed
---maxstam = max stanima






--- Vars
local plr = game.Players.LocalPlayer
local char = plr.Character
local hum = char.Humanoid
local sprtspd = 30
local regspd = 16
local maxstam = 100
uis = game:GetService("UserInputService")
letgo = false
------------------------------------





uis.InputBegan:connect(function(input)

if maxstam > 100 then 
    maxstam = 100
end
    if input.KeyCode == Enum.KeyCode.LeftShift then
            while true do
            hum.WalkSpeed = sprtspd
            maxstam = maxstam - 1 
            wait(0.1)
            print(maxstam)
            if maxstam == 0 then
            hum.WalkSpeed = regspd
            elseif letgo == true then
            hum.WalkSpeed = regspd
            break
            end
        end
    end
end)





uis.InputEnded:connect(function(input)


    if input.KeyCode == Enum.KeyCode.LeftShift then
        letgo = true
        while true do
            maxstam  = maxstam + 1
            wait(0.5)
            print(maxstam)
            if maxstam == 100 then
            return
            end
        end
    end
end)