My self esteem is higher now than it was when I was married. When I was married my exh was the breadwinner, we moved a lot for his career, I was a SAHM with no family support around and I felt pretty useless a lot of the time.
I am now living in my own place, working, raising my DS and enjoying independence. I have realised that I do not need a relationship to define who I am as a person. I have learned a lot about myself in the past few years, what I am and am not willing to put up with. I have a very low tolerance for bullshit and I do not need to be in a situation where I have to tend to a man's feelings as well as my busy life with a 4 year old.
I am not opposed to love and marriage, but it just is not important to me right now. Will it be in the future? Who knows, but I'm living in the present.
Would I be upset if my FWB said he had met someone else? No. I'd miss the set up because it isn't an easy thing to find (I don't often meet men I fancy) but I certainly would not be hurt. I'm a grown woman and I know exactly what I was expecting from this arrangement.
For the PP who asked, I'm in the Midlands so not NI 
A lot of posters on here who are basing their negative opinions on FWB are purely judging it from how they themselves would feel. Different horses for different courses.