BS here. I haven't done any research so please don't attribute any science to this, but I see it as the difference between confidence and respect.
To me, self esteem indicates confidence in your self and knowing your worth no matter what anyone else in the world thinks about you.
Self love is, I think related, but it goes more toward the respect you have for yourself and the behaviors you will and will not tolerate from yourself. If you are faced with making a choice that you know is ultimately not good, part of what should stop you is the respect you have for yourself. In other words, you like yourself too much to put yourself in a bad position and to have to view yourself in a poor light once the choice has been made and you can't turn back..
Does that make sense?
Like anyone, I've been faced with lots of choices over the years. I do believe that self love has stopped me from making some of the very obvious bad ones. I want to think of myself well. I want to think that, while I'm far from perfect like everyone else in this world, at the end of the day I'm a good person in my core.
If you look at it like that, I think you can see where your BSO is going with her concerns. I think she is afraid that the feelings you have for her will be your sole motivator to never stray again. As we all know, those feelings ebb and flow in long term relationships. When you hit another ebb one day, she wants to know in her heart that there is more there and more inside of you that will stop you from ever making that choice again.
ETA - how to develop self love? That I'm not sure about, but if you are not in IC, I would explore that possibility. An IC can help you figure out where the feelings of low self esteem and love come from and how to change that. Everything stems from somewhere and sometimes it's real hard to see without professional help.