Help me write and understand a GLSL reflection shader

Job ID: 35261257

Budget: $10 – $30 USD

I have tried to write a reflection map shader in GLSL and it does not look right.
There are examples of reflection shaders online but they are usually for 3D and more complex cases.

In my case:
1. It is only a 2D fragment shader (no vertex shader)

2. I do not have a 3D mesh. Instead, I use a normal map to get the normals.

3. My environment map is just a 2D texture. I only need reflections from in front of the scene and not behind it

I have attached a tiny HTML project with an example.
Related categories: 3D Rendering OpenGL Graphics Programming Three.js