How you feel inside is usually reflected in your words and actions, so if you constantly feel inadequate then you would always seek other people's approval and usually won't deal well with compliments. Eventually your confidence level drops so much you may even become depressed.
In any relationship, we love to know the other party is someone that is capable of making their own decisions and holding their own. Confidence is the one thing everyone needs in any relationship. It shows you have your own personality and are comfortable in your own skin. Being clingy and always seeking the other party's approval can be annoying and a turn off for most people.
To fix a low self esteem, you have to be ready to honestly examine yourself. Identify what the issues are with you. What are the things that you feel most negative about? Is it a part of your body? Culture, education etc. try to accept and embrace who you are, concentrate on the positives and not the negatives. What are the things that make you happy? Do them most often and don't do things because other people are doing them, do things because you genuinely want to.
Finally, love who you are, other people cannot love you if you don't love yourself first and if you love yourself you would do things that would make you happy which would be reflected in your everyday life and ultimately your relationship.