Ooh, hard one to answer. I kind of know where you're coming from as I'm generally considered very intelligent and yet I appear to be incapable of understanding some mechanical concepts, however much I try. It's like a complete blockage, so if that's across multiple areas (and everyone around you seems to be doing better) it must be hard to cope with.
It seems odd that you could really lack that capability in every area, though, especially given you are interested in learning, which is at least half the battle. A couple of things spring to mind, but this is not my area of expertise at all - one is a weak working memory. I think this means that it's hard for you to hold multiple different things in your mind at the same time, in the way you need to be able to solve a problem or understand something complex. Did you have any learning ability diagnosis or assessment at school?
Another possibility is that there's a psychological element here. You are certainly very hard on yourself. In your post you express yourself well, get your point across, and your spelling and grammar is fine, so you clearly are good at something! Sometimes people develop a belief that they are bad at something, which makes them fear it, which means their brain is full of that fear and dealing with that rather than on the topic they're studying it.
I wonder if you had perfectionist parents, or your siblings seemed more obviously bright, and your parents played that up, meaning you were left feeling like the stupid one. Any of that could make things seem worse than they really are (i.e. your low opinion of your abilities is inaccurate) or could actually cause a mental blockage that makes it hard for you to learn.
If it is something that is holding you back or upsetting you, then it might be worth seeking help. I don't know what exactly to suggest, I'd probably try cognitive behaviour therapy first to see if it's negative beliefs that are causing the problem, before going through difficult/expensive learning disability assessments. However hopefully someone like an educational psychologist will be along to advise.