Question: I don't know how to put it. I have never published before. Previously my one paper was rejected, that was because the experiments were not ready and my advisor asked me to still submit it just to have practice.

Now, 6 months later, I am writing another paper (on a similar topic). I have done some experiments, some are still going on. My advisor had advised me that "even if your experiments are not complete you should still start writing the paper". 

But I'm doing a lot of procrastination and I think I am scared of writing and possible rejection. I have done a lot of writing in other forms before, like blogging etc. But the thought of writing an academic paper with all the stringent rules (everything has to be clearly written, cited, nice flow of thoughts) is scary.

Please let me know how should one's mindset be while in the process of writing a paper.

Answer: 
Like anything else in life, perform the task with the goal to succeed, and the likely outcome of failure.

Understand that rejection is all a part of the game. Very few people succeed more than they fail. As a matter of fact, upwards of 90% of all academic papers are rejected for one thing or another.

The most important thing is to keep track of mistakes that you've made with past academic papers. Write them down as bullet-points on a separate sheet of paper. Keep that paper right in front of you as you work on other academic papers to remind you of what **NOT** to do.

Practice makes perfect. If you're just starting out, it's likely that you're going to have many failures before you have a successful one.
Rejection can be hard to handle, but a good motivator is to look at people who's work was rejected initially widely, but ended up being immensely popular and profitable. Ghostbusters was rejected by three movie studios. Frank Herbert's manuscript for Dune was rejected by over 20 publishers. Lorenz's seminal paper on chaos was rejected several times if I remember correctly.