Article Keyword - SERPs Ranking (Page cannibalization?)

Hi, I have a question regarding some articles of my website. I am currently trying to rank a page higher in search results. To do so, I am writing related content. I'll take an example to illustrate it.

Let's say I want to rank for the keyword "SEO practice". I first create a page with this keyword in mind. I then use 'Google Fetch' to directly have it in search results.

After a few hours, I see my page ranking at the 60th position. After a month, the page remains in the spot 55-60.

Afterwards, I decided to create a blog post with the keywords "Top 10 SEO Practice" in mind. Then, I am fetching the page again and notice a decrease in page rank for the keywords "SEO practice" because Google finds this new blog post more relevant for this keyword.

Therefore, by creating a blog post related to the former page topic, it is competing against each other. Moreover, I am not able to find the page targeting "SEO practice" anymore when typing SEO practice on Google. I will only see the blog post.

My questions:
Is it good to create related topic blog post?
Is there only one page/domain ranking for a particular keyword?
Any advice?

Thanks in advance for your answers

Make some pages 'Corner Stone' and internal link to them from your blog. This should help you rank your page rather than your post (I know this doesn't answer your question but I hope it helps)