Well, I can relate. I don’t know that I have ever thought about it as validation, but I have suffered from something that sounds a lot like you describe my whole life.
For one thing, I would try to let go of the whole concept of valid vs invalid. If you look inside for validation, you are still perpetuating the idea that you can be invalid. And besides, our own minds are often a much harsher judge of our “selves” then others are.
BTW: You said “Well I look and look within for some solid sense of self and I can’t find any.” Buddhists would tell you that is because there “Is no self.” As strange and esoteric as that may be, if you look at it hard enough, you will find that to be true, but that is a whole other thread. Google Anatta if you are interested.

Anyway, whether you believe in Self or No Self, the mind has created a character in the “ego” it believes is a pretty good model of “self”, and this is what it is constantly comparing “other” and “world” to. We feel and believe ourselves to “be” our ego, though in truth we are not. When I was first born I had no ego, and so the mind needed to create one to help guide my body through this life experience. At first it did this by looking outside to parents and care givers to try to get some sense of what this “self” is that it needs to protect and hopefully help thrive. And so the foundations of my ego was laid down by others. But eventually we need to grow up, become independent, and make our own judgements about our “selves”, and so the mind must learn how to fulfill this self vs. world judgement role on its own. You might say it needs to learn how to parent or coach itself, or how to judge, shape, and mold the ego on its own. One important function that the mind and its ego serve is to compare “self (i.e. ego)” to “other” or “world”, determine where “self” may be “deficient” and generate “shame” to motivate change and improvement of “self”. Humans are a social animal and depend on society for survival more so then our own teeth and claws. Consequently much of this function is going to be preoccupied with judging “self” and its place in “society”. This is not wrong, but rather the mind/brain’s way to help “self”, survive, improve, and thrive.
The problem I have, (and maybe you) is that my parenting role model relied extremely heavily on this critical judging model and so my mind learned to do so as well. It becomes over critical and overbearing in it’s attempt to be protective. It becomes a constant game of comparing “me” to the “world” and “others” and looking for ways to improve and move up. The mind is always looking for critical flaws to protect, looking for things I need to change, improve, or hide. This is just the mind doing its job (one of them anyway) as it has learned to do it, but it literally drives itself mad by never letting down its guard and never accepting (validating?) who it currently is. It never sees itself as good enough. BTW, it tends to do this to “others” and the “world” as well, which can make Johnny a real cynical drag to be around sometimes.

One helpful thing for me was to study the concept of the ego and come to an understanding of what it really is and why the mind created it in the first place. Another thing that helped was to gain an appreciation for just how inaccurate this “ego” representation of a “self” that the mind created is. The ego and the “self” critiquing and criticizing mind are still there, but now there is another voice in there that knows what they are and not to take them so seriously and literally. They are still a part of my life experience and that’s ok, as long as they don’t take over the whole show. I don’t try to shut them out, as this just makes them shout that much louder. I pay attention to them, acknowledge that they may have a valid point, and appreciate what they are trying to do for me. This sounds a bit odd, but when it comes down to it, none of those little voices in our heads (so to speak;) appreciate being invalidated.
When you are ruminating about your “self” and comparing your “self” to “others” or the “world”, and pointing out where you need improvement, you are parenting your “self”, molding and shaping your own ego. Our being parented does not stop when we grow up, the mind just takes it over for “ourselves”. Ask yourself this. Now that you know how awful this feels, would you recommend parenting a child by constantly comparing them to others and the world and pointing out where they don’t measure up and driving them to “shape up”. Yet this is what I learned to do to and for my “self”, and my mind believed it was doing a good thing in doing this.
The lesson in the end (IMHO) is that we don’t derive validation, self-esteem, or self-worth from others or from examining our “selves”. They simply are there, inherent and immutable, we just need to recognize them as such, for our “selves” and “others”.