Hello Lee.
We all continue to grow, and some have more insight about it than others. I think you are going through a phase of rapid personal growth at the moment, and along with it you are gaining insight at a faster pace than feels comfortable. This is not unusual, and sometimes triggers major life changes, sudden awareness of who we are, our perceived limitations and where we might best be headed.
It can be sobering, taking our heads out of the sand about ourselves. It often heralds the uncomfortable realisation that we don't quite fit our long-held view of ourselves. It is called growing up, and sometimes takes decades. Eventually though we do usually come to terms with the reality of who we are no longer influenced by other's judgements (which are all a reflection of them, not us) and that insight, when it comes and when we are over the adjustment period, is very freeing. (Those that don't might be called sociopaths or narcissists or fit into some other box determined by psychiatrists or psychologists or the media who don't take into account the vast and all normal range of human personalities).
Re the learning. You are not thick. You show great insight and a refreshing honesty. You are on a journey to self-awareness that will enrich your life. Lots of people (like another poster said) learn through doing - my H for one. He won't read or watch or listen, he just gets on and does, and that works best for him. He's brilliant at practical things and as a musician (self-taught, through playing) yet was written off at school. His brilliance at practical things could make someone else whose strength might be IT, for example, look incompetent. Me, things told to me are in one ear and out the other. I am short-sighted and through all the years spent at the back of the class looking out at a blurred scene with some general babbling going on around me, I learnt nothing. As an adult I discovered late that I learn by seeing - reading and writing and doing.
Your current re-running of your early life and examining all the tiny details that went wrong could possibly be a sign that you are depressed. I think whether you are depressed or not it is a sign of growth.
Questions to ask yourself. What do you enjoy doing? What are you good at? I would say writing, for a start. The fact you are writing in a journal and are having sudden insights is a great sign. Keeping a journal is frequently recommended to help with depression and is also the way many writers start.
I think you are being very hard on yourself. You sound like a nice man to me, on the start of an important journey full of uncertainty but ultimately worth every challenging step. You are showing great insight. You have a university degree (a huge achievement). You have a job.
Sometimes our problems can be as simple as trying to be a square peg in a round hole. Society or our upbringing keep trying to squeeze us into somewhere that is just plain wrong for us. Find out what you like, where you fit. Where your strengths lie. Find your people, your Lee-shaped world. It is out there, Lee. Maybe try volunteering in something that interests you. There are so many openings for everything from hedge-laying to soup kitchens to riding for the disabled. Volunteering might help build your confidence, and through helping other people you may find it helps you. You will certainly meet a wide variety of people from all walks of life.
I do think it might be helpful to talk things over with someone. Does your job have access to counselling? Many do, and it is always confidential. Or you could try your GP surgery.
I seem to have wittered on a bit. Your post must have struck a chord. Good luck.