How do I fix this textbutton GUI brighten script from spazzing?

So there's a play button. When the player's mouse enters the play button, the play button gets brighter. When it leaves, it returns to its original green color by un-brightening. I've made a pretty good attempt at this, but there's one issue: when the mouse quickly enters it and quickly leaves, it alternates between brightening and un-brightening and results in a random color.

Just in case you need a visual, look at this GIF of the problem: http://i.imgur.com/dvY8WBh.gif.

Another: http://i.imgur.com/ugLFhrv.gif.

Here's a GIF of it working correctly: http://i.imgur.com/wasU1Gy.gif Notice how when the mouse enters, the GUI brightens. When the mouse leaves, it un-brightens to its original state.

And finally, here's the code:

complete = false -- used to check if the gui is done brightening
in_session = false -- used to know when to unbrighten 
left = false -- used to know when the mouse left the gui

textButton.MouseEnter:connect(function()
    brighten = coroutine.wrap(function() -- this function brightens the button
        for i = 1,37,4 do
            in_session = true
            textButton.BackgroundColor3 = Color3.new(i * 1/100 + textButtonR,i * 1/100 + textButtonG,i * 1/100 + textButtonB)
            wait()
            print(i)
            current_color = i --used to unbrighten at any stage of the brightening process just in case the player's mouse leaves while it's brightening. allows for unbrightening even if it's not 100% done brightening. 
            if i == 37 then   -- brighten function is done when it reaches 37
                complete = true
                print ("complete")
            end
        end
    end)    
    brighten() -- brightens the button when the mouse enters
end)

if left == true then -- if the mouse leaves, the brightening stops 
    brighten = coroutine.yield()
end

textButton.MouseLeave:connect(function()
    left = true -- if the mouse leaves, this variable is set to true 

    local unbrighten = coroutine.wrap(function() -- unbrightens the button to its original color. 
        for i = current_color,1,-4 do -- used to unbrighten if the mouse leaves during any stage of the brightening process.
            textButton.BackgroundColor3 = Color3.new(textButtonR+i/100,textButtonG+i/100,textButtonB+i/100)
            wait()
            print 'left'
        end
    end)

    if in_session then -- if the mouse left and the brighten function is going on, then unbrighten the button.
        unbrighten()
    end
end)

If you have any questions, I'll be happy to clarify. Thank you so much if you are able to solve my problem. By the way, this is not all of the code. I have omitted the code used to make the GUIs. If you want that, here's a pastebin of all code: http://pastebin.com/NKiqR0S8

EDIT: So I have managed to fix the original problem myself with some help from CreativeEnergy from the community chat. But now I am faced with a different problem: the button does not get brighter if the mouse enters the button while it is un-brightening.

GIF of the problem: http://i.imgur.com/yVb1bHv.gif Notice how when the mouse enters the button while the button is un-brightening, it doesn't brighten. But when it leaves, it unbrightens.

Here's my updated code:

complete = false -- used to check if the gui is done brightening
in_session = false -- used to know when to unbrighten 
left = false -- used to know when the mouse left the gui

textButton.MouseEnter:connect(function()
    brighten = coroutine.wrap(function() -- this function brightens the button
        for i = 1,37,4 do
            if not left then
                in_session = true
                textButton.BackgroundColor3 = Color3.new(i * 1/100 + textButtonR,i * 1/100 + textButtonG,i * 1/100 + textButtonB)
                wait()
                print(i)
                current_color = i --used to unbrighten at any stage of the brightening process just in case the player's mosue leaves while it's brightening.
                if i == 37 then   -- brighten function is done when it reaches 37
                    complete = true
                    print ("complete")
                end
            end
        end
    end)    
    if last_color == 1 or last_color == nil then
        brighten() -- brightens the button when the mouse enters
        print('yeah')
    elseif 1 < last_color then
        unbrighten = coroutine.yield()
        print('ok')
        for i = last_color,37,4 do 
            if not left then
                in_session = true
                textButton.BackgroundColor3 = Color3.new(i * 1/100 + textButtonR,i * 1/100 + textButtonG,i * 1/100 + textButtonB)
                wait()
                print(i)
                current_color = i --used to unbrighten at any stage of the brightening process just in case the player's mosue leaves while it's brightening.
                if i == 37 then   -- brighten function is done when it reaches 37
                    complete = true
                    print ("complete")
                end
            end
        end
    end
end)

if left == true then -- if the mouse leaves, the brightening stops 
    brighten = coroutine.yield()
end 


textButton.MouseLeave:connect(function()
    left = true -- if the mouse leaves, this variable is set to true 

    local unbrighten = coroutine.wrap(function() -- unbrightens the button to its original color. 
        for i = current_color,1,-4 do -- used to unbrighten if the mouse leaves during the brightening process.
            textButton.BackgroundColor3 = Color3.new(textButtonR+i/100,textButtonG+i/100,textButtonB+i/100)
            wait()
            print 'left'
            last_color = i
            if last_color == 1 then
                left = false
            end
        end
    end)

    if in_session then -- if the mouse left and the brighten function is going on, then unbrighten the button.
        unbrighten()
    end
end)