Check out Melanie Beattie "Codependent No More"...has exercises to help walk you through this and is laid out to be very user friendly. I used this book years ago to take my first steps away from codependency....

I'm not co-dependent, but I've dealt with a lot of people in my life who are/were.

Please correct me if I'm mistaken, but part of dealing with it is recognizing that you don't need someone else to feel complete by yourself - and maybe talking yourself up a little might help. Say positive things to yourself in the mirror.

Much like hearing negative things about yourself will eventually wear you down, hearing positive things about yourself will eventually make you believe them. Notes in the mirror, Notes in your lunch bag. Notes in your car. Do things that make YOU happy. Learn a new skill. Pick up a new hobby.

Understand that you cannot make someone else happy with you if you are not happy with yourself

A book that's good for a lot of situations is "The Gift of Fear" by Gavin DeBecker.

I am a reformed codependent doormat fixer facilitator martyr. *waves*

And I had no idea until I was in my forties. Grew up in a family with constant low-level chaos and a mother who was often upset. SHE was a doormat enabler, and I was the good girl who tried to comfort her.
I worked in middle management in the hospitality industy, where success depends on pleasing others. Got promotions because I pleased higher ups by shoring up the weaknesses of my immediate supervisors. Then I married a man who had lots of problems from a family of multi-generational dysfunction - yay, another way to avoid my own issues by focusing on those of others!

Two things that really helped me were reading Codependent No More, and happening across a diagram while reading about narcissism (OSD is a narcissist). I wish I could find it again, because it instantly made me see my own problem. It presented narcissistic traits across a spectrum, with full blown narcissist on one end, a healthy personality range in the middle, and - you guessed it - codependent at the opposite end of the scale. It was a revelation to me that A) my problem had a name, and in its own way was equally as bad as being a narcissist. That was when I stopped feeling bad about putting myself first.

We are all responsible for ourselves, then for our offspring. We are not responsible for our SOs, their choices, mistakes, baggage, relationships, etc. It's so very important to know what our role is and is not, to stay in our lanes even when pressured to step out, and to maintain healthy boundaries that help promote healthy relationships.

I'm still a work in progress, but am much better at recognizing those codependent impulses when they arise and dealing with them appropriately.

There is also a 12 step group called Codependents Anonymous. There are groups in most communities. Many people with codependent tendencies find it helpful to attend meetings and share their experiences. The organisation publishes its own literature, and as others have mentioned, there are several authors that have written some good stuff. In addition to Melody Beattie, I like Harriet Lerner (Dance of ... books).