I find it interesting that the "thinkers" are referring to love as a feeling, and I'm a "feeler" over here referring to love as a choice/commitment/acceptance thing.
I think truthfully it has to be a combination of both or it will be lacking. Love that is based on only feelings will not last because feelings fluctuate. And choosing to be with someone everyday and fully accept them, even if reciprocated, will be lacking if there is no feeling/deep connection... I know I'm kind of stating the obvious here.
Obviously, I suffer from depression. Anxiety is an old friend, but the veil of depression started clouding my vision just 2 years ago. I'm 31 years old... that's kinda fucking weird to find yourself suddenly dealing with depression. Most depressed people I know are seasoned pros, so to speak, so it can be awkward and scary to find yourself suddenly lacking resiliency and self-esteem/worth/acceptance/love/insert whatever term necessary at a not-so-young age. I always considered myself a *very* resilient person beforehand and never lacked confidence in my relationships. Then, I married a wack-job who basically destroyed that part of me, and learning how to rebuild it has been by far the biggest challenge of my life. Ironically, I'm actually quite attractive by many societal standards, but as with loving others, it takes more than just good looks to love yourself. I also think that maybe I have a touch of "ugly duck syndrome." I digress.
What else besides feelings and "choosing" is crucial to love? I think my answer would be trust and respect. So, maybe it's not as much about "loving" yourself, because as we've established, it's difficult to even define love. So, maybe it is more about trusting and respecting yourself. If I don't trust myself, how can I trust that my feelings for my partner are real? How can I then trust that my partner's feelings for me are real if I'm feeling ambiguous about it at times? If I don't respect myself, I probably won't have solid boundaries set in place and I will probably be unable to respect my partner's boundaries too.
I still maintain that in addition to the almost undefinable feelings of love, it boils down to acceptance. "I see your missing pieces, and I still love you." This can be applied within or to others. Think about it... what happens when you don't accept something? You react. You get defensive. You may even lie or lash out in anger. These are things we've all done with ourselves and others, and they are the ultimate love killer.
I'm also 31. Had a severe bout of depression and anxiety about... 6-7 years ago, lasted for about 3-4 years. I'm on the other side of it now - awareness of my tendency to slide in that direction makes me take precautions against it. It's not at all related to age, but to what I've bolded. I too met someone who took away my sense of agency and identity, destroyed my self-esteem to the point where I questioned if I deserved to live. It does take a while to rebuild but you will gain a lot of resilience and self-knowledge from this.
Before, I too thought of myself as a really "tough" person. I'll borrow a metaphor to describe this. There's two types of toughness - a brittle toughness, like an oak tree, all stable, able to take a lot of hits/abuse, and it's still there after a hundred years. Then there's a whippy, young willow tree, that bends in the wind, roots holding firm. When a hurricane blows in, no matter how strong the oak tree is, in the face of 100mph winds, quite a few get uprooted. The willow tree bends in the wind, its flexibility allowing it to stay grounded, stay with its sense of self. You don't find the toughness that you lost before, since you can't go backwards and regain ignorance. Instead, you build a new resilience that holds you down even as everything whirls around you.
That's where the questions come from about your worth, and "loving" yourself. You need to find what's important to you again, look for things that you know are a part of you, and believe that all good and bad, are things to accept and appreciate. When someone continually points out the bad sides of you and makes you feel "less than", you train your own mind to look for the same things, and ignore the other sides of you that complete who you are. What's commonly known as "loving yourself" is choosing to focus on all aspects of yourself, to see your personhood as a given, your identity as a work in progress that is worth fighting for. This also relates to what you wrote about trust and respect. The only way to regain trust in and respect yourself is to commit to working on yourself, and observe yourself proving the nut-job wrong.
I also firmly believe that there aren't parts that are "missing" in anyone. I've struggled a lot with that feeling in a personal context. What we see in one context as "missing" is what gives us abilities/perspective in other areas. If we were to compare a person, a life, to a painting, the dark bits - the "missing" bits are as necessary to the whole picture as the bits that have colour. There would not be a coherence or a beauty without both. Our eyes are alternately drawn to the dark or the colour depending on where we are in life and what's happening around us. But both are always present, and both are always necessary. When we're depressed, all we can see is the darkest parts. There are some who will take advantage or manipulate to make it all that we can see. That creates a dependence on them for a sense of worth. That's the basis of co-dependence. There are also some who will tell you "just look at the light" - that's not helpful, because your brain is currently trained to see only the dark. Self-care and "self-love" is hard work and will take consistent effort because it's essentially re-training your brain to "zoom out" of the dark aspects of life and yourself to see the whole.
As for how it relates to "love" in the sense that you're describing, by knowing who you are, and having belief in yourself, you're less susceptible to manipulation. When others criticise who you are "inside" and drag out details and bits of "evidence" to support their case, you know that it's untrue. Or you will be at a point where you know that it's true, and you're like "so what? this supposed failing also enables me to do x, y and z, it gives me value". It's a form of protection that allows you to identify who is good for you, who helps you to grow/supports you and who is a manipulative nutjob. I don't believe that loving people is acceptance "even with" their "missing" bits. I believe that loving people is being able to see the humanity and potential in the supposed "missing" bits and encouraging us to express those aspects of ourselves in healthy, constructive ways.
*edit: Sorry the grammar's kinda all over the place. It's personal, obviously, and I often struggle with words for such stuff.