Take notes. It's the only way I'm able to hang onto specific information and concepts ( ie code syntax, structures, processes ) in any reliable way. Your own notes are infinitely more valuable than any textbook(or blog or forum or whatever). Your own notes will be in your thought patterns, meaning when you read them later the information is 'ready-to-eat'. Textbooks written by someone else provide information which first needs to be wrestled into shape before you can use it.

I have a self-hosted nextcloud server. I spent weeks learning how to set up apache, SSL certificiates etc. Then when 3 months later something broke, I had to learn it again from scratch - which led to me writing 'guides' for myself for all the stages of the process so the next failure would be easier to recover.

If it's worth remembering, it's worth writing down.

All the best! New languages (human or machine) are always difficult, but incredibly rewarding.

Unsure if it'd be your case, but maybe you're too tired? Tiredness can mess memory afaik. Or maybe you're not using the ideal learning methods? In my case for language learning, for example, repetition is a guaranteed way for me to lose focus, instead with cinversations being ideal for learning.

i write millions of notes for everything.

to that i add a similar amount of reminders.

not just for learning etc.

also for things like the first todos of the next day: shower, prepare food, pack this, ...

for remembering for short-medium timeranges very simple notes suffice.

i know lots of stuff. i just can't access it without a trigger.

How much of a brain break are you giving yourself?

If you're spending a large amount of time on learning each day, you might look into structuring your learning so you're doing it in small segments each day. Learn for 30 minutes, then go do something mindless for an hour.

Since short term memory is largely written to long term memory while you sleep, maybe try working on it an hour or so before you go to sleep.

Also, how are you learning? Are you doing exercises as you learn each concept, or are you reading a bunch then trying to throw everything together after?

For me, the only way I retain information like that is to actually work with the knowledge. Taking notes, like many have suggested, doesn't work for me. I learn best when I have a task I want to accomplish, and I have to go research what I need to complete it.

Guided learning (school, training courses, books) really doesn't transfer to long term knowledge for me, so I structure my learning differently. I feel like this is a pretty common thing with ADHD.

Something that I haven't seen in the comment section yet is having a diet that supports your brain functions.

(I want to preface that in no way do you have to start a new diet for you to feel improvements. But it sure helps to build it out fully. Also, I'm using the word diet as in what you eat to sustain and improve your body, brain, and the systems within. And not one of those strict, fad-like things.)

Starting my day with something high in protein, healthy fats, omega-3s, minerals, fiber, and if I can, antioxidants, is a must. I personally love eating a yogurt bowl for breakfast every day, as it provides all of the above. Mine consists of 4 Tps of plain yogurt (meaning no flavor, added sugar, or additives), 2 Tps of low sugar count flavored yogurt such as peach, vanilla or strawberry yogurt, 3/4ths of a cup of rolled oats, 1/2 - 3/4ths of a cup of low overall sugar and high protein trail mix, 1-1 1/2 Tps of chia seed and 3/4ths to 1 cup of no added sugar almond milk. And if in season, I absolutely add washed blueberries to my bowl. The trail mixes I use all have deshelled sunflower seeds as their mix. With peanuts, almonds (whole, halved or slivered), cashews, pumpkin seeds, raisins, and sometimes semi-sweet chocolate chips as well.

Where's what we should be focusing on with our diets as people with ADHD:

Protein for Neurotransmitters

A consistent recommendation is to start the day with a protein-rich breakfast and include protein in snacks throughout the day, because protein supports the production of brain neurotransmitters essential for focus and attention.

Healthy Fats and Omega-3s

Regular intake of omega-3 fatty acids (from sources such as eggs, salmon, almonds, walnuts, avocados, flaxseed, etc.) as they may improve focus and support overall brain function for those with ADHD. Fish oil supplements are sometimes suggested.

Reduce Sugar and Additives

Dr. Hallowell routinely warns against diets high in sugar and artificial ingredients, as these can cause blood sugar spikes and worsen symptoms of inattention and hyperactivity. "A diet high in carbohydrates, sugar, trans-fatty acids (and all those other ingredients contained in the many processed foods we Americans love to consume) is not beneficial for anyone, least of all people coping with ADHD." - Feeding Your ADHD Brain... Literally

Hydration and Micronutrients

Proper hydration and intake of key micronutrients are vital for brain health, especially for individuals with ADHD. Staying hydrated helps regulate neurotransmitter production, particularly dopamine, which supports focus, mood stability, and impulse control. Mild dehydration can impair attention, memory, and executive functioning, leading to brain fog, irritability, and fatigue—issues commonly experienced with ADHD. - Hydration and ADHD - Nutrition in the Management of ADHD: A Review of Recent Research

A reminder that I'm not perfect in achieving these things and still struggle in my life, regardless of whether I eat healthy consistently or not. But I can tell you that it has greatly helped me not feel as out of it every day, low energy, in a brain fog, lack of 'average' focus, etc. One other thing to add to this is either taking a multivitamin (that you've researched) or, for me, drinking a Vitamin Water daily. From that, I'm getting vitamin B12, which helps support your energy production. And while it's not definitive in research, higher or normal amounts of zinc (8-10mgs /d) have been shown to suggest that it can help reduce symptoms of hyperactivity and impulsivity. Some studies have shown that having a regular amount of zinc in your body may enhance the effectiveness of stimulant medications. - Finally Focused: Mineral Imbalances & ADHD (Part 1: Zinc Deficiency & Copper Excess)

Hope this helped.

6 months is barely any time if you’re just starting out learning to code. Give yourself a year or two before you should expect to really understand a lot of the core concepts and know them by heart. Right now I’d say you should be able to read and understand basic code, you should know how to write code with references available, and you should be familiar with things like data types, classes, and functions, but not necessarily on a deep level.