In determining whether or not you have PTSD resulting from the events in your life one would not only have to look at what was done to you but how you internalized it. An outsider looking in could not tell you if that was sufficient cause for you to have dramatic repercussions.
That being said I suffered from the same thing for the longest part of my life.. it would seem that I would pay attention harder than everyone else take notes better than everyone else with the fear that I would miss details and others around me would doodle and talk to each other and play on their phones and still get all the pertinent data and I would somehow still manage to upset my boss/parents/teachers because I didn't understand what they had needed.. for the longest time, I lived with guilt and regret over missed opportunities beating myself up on how I could be so smart in some things and so inefficient in other things particularly ones to do with missing details.
I tried every kind of therapy and read every kind of self-help book because I just could not accept "the way I was".. I knew if I just found the right technique I could somehow beat this! Up until a year ago, the only things that helped a little were Tony Robbins, Eckhart Tolle (totally opposite by the way but I took the best of both), the Sedona Method and CBT. Reading the philosophy of Epictitus also helped.. it's along the lines of CBT. It wasn't until I was introduced to the idea of radical self acceptance that I began to understand how true transformation takes place.
Basically the idea is that "Whatever you try to control, controls you." However, when you accept fully the parts of you that at currently unacceptable to you as your fate, then miraculously they start to change for the better. This seems paradoxical at first and is rejected by your Neo-cortex/your thinking mind. But the reason it works is because your habits, patterns, heck entire personality is held in place by opposing emotions energy efficiency e.g addiction to alcohol and the opposing guilt. Without one, the other can not exist. However, usually only one is allowed to come to the surface while the other is repressed. If both are allowed to come up to the conscious mind and accepted without judgement, they dissolve and open up space for change.
In your case, your brain is causing a block in your ability to learn things as well and as easily as it could because in the moment that you were belittled, your emotion was split in two. You either had the choice to
1. Believe your stepfather and make his words true but still believe that what was happening to you was caused by you thereby giving you some control.. that if you just tried harder you could control the situation
2. Believe that your stepfather was wrong but you still were subjected to (what you perceived as) abuse. Meaning you had no control. Helplessness is one of the MOST painful emotions for the a human and most animals to accept and they will do anything to avoid feeling it. That is because it is interpreted by your primitive (reptilian) brain as imminent death. So you took the "better" option at the time and made true the prophecy that you were a slow learner.
Mindfulness will really be helpful in your case. Also you need all negative opinions about yourself to come to you uncensored and accept them fully. Give your "imperfect" qualities the unconditional love and acceptance that your caretakers did not give you growing up. Be there for yourself and feel the raw emotion that comes with feeling those emotions. But with a quiet mindset also understand that emotions are not representatives of reality but rather a flow of energy at the current time and when allowed to be, take a different path, transform and sometimes leave forever. Along with the emotion also transforms the qualities, skills and urges associated with them.
Couple of good reads are:
1. The one thing holding you back. Rafael Cushnir
2. The way to vibrant health. Alexander Lowen
There is a really good mental exercise to do on YouTube that would be particularly good for you.
https://youtu.be/2jUcdEJ_iZMI completely transformed that exact challenge you have in less than a month with the techniques in this video and doing the exercises in "the one thing holding you back". In short, I did somatic experiencing and it ended up helping with other things I was not even trying to "fix".
Sorry for the long answer but I really wanted to help share this with you because I know what it's like to feel the way you do.. I did for the better part of my life. But it's important for you to know that the human spirit is very powerful. It will amaze you with what it can do for you. But it has to start with giving more love and acceptance to your dark side than you do your positive side.