Does this happen when you interact with specific individuals or is it a feeling that just pops up and you name it as "unlovable"?
Thank you for clarifying. It seems that you've tried therapy. Therapeutic modalities will work so far as much as you allow yourself to change your internal dialog.
Hypnotherapy and NLP (neurolonguistic programming) are quite helpful in changing though patterns. Hypnosis targets the subconscious and that way it bypasses your conscious self critic. It also takes only about 6-8 sessions to address self esteem issues. What you're struggling with to a large extend has to do with self perception and what you believe (it does not have to be realistic. It is just what you tell yourself over and over)
There are other self help tools like CD or subliminal messages as well as autogenic training.
In your life, you will encounter people who love you and who do not. Self worth will always come from within no matter what.
Try to meditate and come up with an answer to - what does love mean to you? Is it acceptance, unconditional regard, positive feedback, absence of judgment, is there a specific part of you that does not feel judged (your inner child, your ego)etc.
Train Your Mind, Change Your Brain: How a New Science Reveals Our Extraordinary Potential to Transform Ourselves
Self Hypnosis - Believe In Yourself & Increase Self Esteem by Secret Changes Hypnosis (MP3)
Love (Guru in Jeans: Inward Journey to Psychospiritual Awakening)