When you were a kid, did your mother ever say What will people think? I have a feeling many of us heard that when Mom admonished us about a misdeed. Lesson learned was that we need to worry about what others think of us.
To a certain extent, that's true. But , like the poster says, it's what you think of yourself that counts most. A lot of what you think of yourself revolves around the type of person you are. Positive people tend to have better images of themselves. Those who put a negative spin on many things in their lives probably view themselves poorly. That's only one part. There's more.
If you don't think that you've accomplished a lot in your lifetime or feel you are always the one on the bottom of the pile, take heart. It doesn't have to stay that way. Pour yourself a cup of coffee or a nice cold glass of tea and make a list of the positive things you admire in other people, other writers if you are a part of that world. Make it as long or short as you like.
Now, check off the ones that you can apply to yourself. Be honest, no one else is going to see this list. It's only for you. Make an objective assessment. Look at the ones that are left. Take them one by one and ask yourself what you can do to change, what you can do to achieve this particular positive. Keep your list where you can see it daily. Are you making any progress?
Change involves effort--sometimes a great deal of effort. Change is seldom easy. If you want to rewrite your self-image, it needs to happen in small stages. No overnight miracles. We make progress in small steps. If you're a writer, you need to sell yourself to editors, publishers and readers. Thinking well of yourself goes a long way in being successful in that sales area.
There's also the possibility that you're very satisfied with who you are. That's great, but you still need to make a concerted effort to keep it that way. How do you feel about the statement in the poster?