I have my keywords showing in the SERP and dropping, whats the issue?

As the title says I am a noob. My site is 3 months old. The domain also same. I have 3 main articles(long form) on the site like 4k 3k and 1500 words. I also have a blog on the site. Only 2 posts are in the blog.
Search console shows my site right now ranks for 150 kws.
But, I have an issue, my Kws are showing up in the SERP and then dropping.like this
I don't know what the problem is? onpage SEO?
Help me