I can't really offer advice marc except to say that I think you are being very honest about a problem which is very widespread. I only first became aware that I wasnt the only one who suffers from this sense/fear of being totally unloveable when my bro and I had an important tete-a-tete a couple of years back. He has been married twice with two sons but he expressed very well exactly the same thing that you have here: He never believed that either of his wives really loved him.
It's the same for me: if someone does say that they love me, I instinctively disbelieve them. The very phrase makes me go quite hollow inside. For me its a little worse than that because it applies to anyone who says anything nice about me: (inside I say to myself: 'if you only knew then you wouldnt say such foolish things.'

Over the past ten years I have had a very strong relationship with someone online and it was really this relationship that began to help me overcome this terrible burden of unloveability.
Having said all that, my own feeling is that this is not a problem that can be solved by another human being however much they might actually love you. Human beings are not designed to love in the way that human beings actually need to be loved (except perhaps parentally) and so at that point the problem becomes a spiritual one which can only be overcome in a spiritual way and so there I will end because I know that this is not an option for you . . at present anyway. The only thing I will try to say is that love has an existential core to it that cannot be gotten around - even by the most avid atheists.
The phrase from that song 'everybody loves somebody some time' is true but it isnt enough for any of us because we need to be loved all of the time, regardless of all our faults and all our whimsies. It's a tall order.
I'll conclude by saying that if we do not love our own self we can never believe that anyone else might so it becomes a personal quest I think: learning to live with who we are - which is not so easy as it sounds. That's really what I mean by 'spiritual' I suppose.