Franklin had a pretty interesting way of improving his writing, which can be generalized. He would read articles that he liked, and then try to rewrite them from memory. That way he could compare what he wrote with the original - which creates a very clear feedback channel.

By comparing your choices with those of an experienced writer, you hone in on exactly where your mental model of writing needs improving. Depending on what you need to improve you can tweak Franklins model. You mention structural problems: to improve you could try to outline good blog posts from memory, and compare what your outline with the original. Where is your pacing off? How many examples do they use? Do you lose the thread? By comparing what you do to the original the nature of your problem will become much more clear, and you will have begun forming deeper, more detailed mental models of how to structure a blog post. You will start to notice classes of openings, different structural principles, etc.

You could approach headlines similarly. Ask a friend to email you sections of blog posts, and give them the best names you can think of. Then look at the original. Try to analyze why the original is better, and try to tease out how the author approaches headlines.

Here's Scott Alexander's tips: https://slatestarcodex.com/2016/02/20/writing-advice/

If you want feedback on your blog then there are four options:

1. You can review the work yourself.
2. You can publish it online and read the comments.
3. You can join a group where writers share drafts with each other.
4. You can hire someone to read your work.

#1 only works if you can figure out what is wrong, which you can't. Besides, you already have published 50 posts on your blog (and written who-knows-how-many drafts) over many years without making progress. #2 tells you useful information but not how to structure a post. #3 might or might not fix your problem. Not everyone's personality is suitable for writing groups. #4 makes sense if you have money and can find the right teacher.

I have trouble deciding how to structure posts (or whether I should split them into a series), and I'm terrible at writing introductions and conclusions. I'm also not particularly good at headlines, although I consider this a less-important problem.

I read the five most recent posts on your blog. Your self-assessment is correct. Your posts do have structural problems. Your introductions and conclusions are terrible. Your headlines aren't great either. These issues mostly stem from your posts' lack of a clear objective.

• "Etherium GPU mining for paranoid noobs" is written like story but the meat of the post is instructional. The objective (what the reader is supposed to get out of this) is confused. You should turn it into a story or you should turn it into an instruction manual.
• "Indexing and sorting to find data quickly" could be better than it is. The sentence-level writing is fine. The technical information is fine. The post is unclear about what value the reader is supposed to obtain by reading the post. Your post should be a systematic tour from low-level to high-level but it reads more like list of $n$ things. One level should flow naturally from the last by solving a problem in the previous level. The introduction and conclusion of this post are bad because they digress from the meat of the post.
• "How to write examples for documentation" has an unambiguous objective. It needs at least one example. Multiple examples would be good. A single example would be better. The post should start with a bad example and then show the user how you iterate the bad example into a good example. Delete the last paragraph.
• "Easy mistakes when writing OCaml C bindings" does use examples. If the mistakes are random then it should be a list of $n$ things. The post shows more promise than a list of $n$ things. Instead of listing mistakes you should structure this post as a sequential checklist of how to debug your OCaml C bindings.
• "How to use Core.Command.Param" should be written either in the format of random access technical documentation or as an introductory guide.

I have lots of experience teaching things and I am available for hire. If you would like to hire me to help you learn how to write better then PM me and maybe we can work something out.

I took this course by Venkat Rao and Sarah Perry and it is really great:

 https://ribbonfarm.teachable.com/p/the-art-of-longform

The Writing Well Handbook by Julian Shapiro seems solid. I've only skimmed through it, but in skimming it looks solid, and I thought his growth marketing guide was really good.

Here's a video of Larry McEnerney from UChicago teaching about how to make your writing more valuable to the reader: https://www.youtube.com/watch?v=vtIzMaLkCaM

Normally, I don't watch videos when I want to learn something, but this one made a big impression on me. I've found the advice it offers at just the right level--not too low to lose sight of the big picture, but also not too high and abstract to confuse me. The gist of his message is to write with the reader in mind. The video offers advice how to do it, focusing specifically on text structure, which may be your biggest problem now.

Edit: Here's a link to the handout that goes with the class: https://cpb-us-w2.wpmucdn.com/u.osu.edu/dist/5/7046/files/2014/10/UnivChic_WritingProg-1grt232.pdf