It is important to remember that self-esteem can fluctuate - and also that it varies in relation to different aspects of life. It is quite possible to have good self-esteem in one area - for example work, - yet be very lacking in confidence in, for example, communication. But generally speaking, problems in a relationship such as jealousy, just not feeling secure or good enough, or assumptions that our partner is being critical of us, or is not supporting us, are often signs that self esteem issues are getting in the way of the ongoing strength of our attachment to each other. It can also make the normal ups and downs in a relationship become much greater, as the person with low self-esteem will often find it harder to forgive after a squabble. Brooding or devaluing their partner can then follow - making recovering from a disagreement much more difficult.
If this is the sort of thing that is happening in your relationship, then acknowledging that there may be self esteem issues at play must not become for you another feature of feeling 'not good enough'. There is an immense courage in admitting ones vulnerability
Low self esteem is learned - in other words, you have probably grown up with assumptions about yourself that you are in some way inadequate, that you don't matter, or that your feelings are wrong. Sometimes these beliefs are handed down through generations. These fears and anxieties are typically based on false ideas about yourself and can create a range of behaviors- ranging from withdrawal to over sensitivity, to a perception of criticism when none was intended.
Sadly too, low self-esteem can allow people to remain in relationships, which are not respectful and at worst are abusive. It is important to work out the role your self-esteem might be playing and the extent to which your partner might be overly critical and judgmental. You say your girlfriend has gone from liking your modesty through to finding your self-doubt unattractive. This movement in a relationship is a reminder of how it can be that that the things that attract us to our partner can also be the things that later become challenging - and makes me think it may well be that the relationship itself could benefit from some attention.
How easy is it to communicate with each other about these feelings you are both having? What is happening to the balance you once had? How supportive might your partner be of your finding a way to address your self-esteem issues? It might be very helpful to you to be reminded about the things she loves about you.
There is a form of therapy called cognitive-behavioral therapy which can help the person with low self esteem. This therapy operates with a form of reality testing and gently challenging a negative, self-sabotaging belief system which you have very possibly unconsciously adopted and which works against you in a subtle and undermining way. This therapeutic approach might be very helpful to you.
Remember to focus on what you do well, on all you positive attributes, inner and outer, and to recall the compliments you have received. Keep interested in your own achievements in life and take very good care of yourself and try to pay less attention to comparing yourself to others.
Overall however, if you feel self-esteem is an issue for you, quietly congratulate yourself on having the strength to recongise this. And make it your business, to find out from a trained professional, a way of getting tools and strategies to manage one of life's very common problems.
"We ask ourselves, who am I to be brilliant, gorgeous, talented and fabulous? Actually, who are you not to be ? ...as we are liberated from our own fear, our presence automatically liberates others" (Poet Marianne Wilson, A Return to Love)