An employee's salary is supposed to be strictly confidential. The fact that you found out what a co-worker is paid is irrelevent. I agree that it is no fun knowing that a co-worker with the same status as you makes more money, but the reality is that in most business, this is a common thing. It is also a big reason why salaries are confidential.
Your job today is probably worth more than it was when you started 6 years ago. The hard truth is that in many cases, companies pay more to external hires than they would for internal promotions. This especially applies to management.
It can also be hard to be the person making more money when another employee finds out. I was an assistant manager at the
Disney Store making $37K per year. Even management salaries were confidential. The other assistant at our store "accidentally" opened my paycheck and saw that I made $10K more than her. When she shared this with the store manager, it was revealed that my salary was $5K more that HERS!. Of course I never knew any of this. But those women hated me for making more money than them.
I had no idea that the Disney Store Manager only made $32K a year. Both of these women had been with the company for years. They worked their way up from CM to management. That is a very slow and a very low paying way to move up the ranks. I was hired as an assistant. Anyway, within a few months, the store manager quit, and I was promoted, and bumped up to a salary of $44K. Of course the assistant quit and even followed the store manager to her new company.
The moral of this story is simple. The best way to improve your salary and earning potential is put your experience to work for you. Find a similar position with another company. Chances are they will pay you more than the 1.5 year person who is now making more than you. Six years at one job is a very long time in today's world. It is also a very good bargaining chip when the time comes to find a new job.
That is how I worked my way from my first management job that paid $8 per hour to an annual salary of over $50K in less than 5 years. Anybody can do it if they have the will to succeed elsewhere. The days of the 10 and 20 year jobs with one company are long gone. Today's reality is more like 3 to 5 years and then you move on.
Don't waste your time trying to get more money from your current employer. You are always going to wonder what the next new person is getting paid.