I have been meaning to ask you for a while but I keep forgetting - this means you are about about to ask, and are explaining why it's been taking so long. I had been meaning to ask you for a while but I kept forgetting - this means you already asked, and are explaining why it took so long.

I have a question for you. -> I still have the question; action is not yet complete => I have been meaning... had = completed action

Thank you so much Paige.

I have been meaning to ask you for awhile.

I have a question for you. I have been meaning to ask you for awhile, but I keep forgetting. If you were to use had, it would imply that you aren't going to ask the question anymore because you changed your mind or you already asked it. Make sure you use have with keep and had with kept. One is present and one is past.