you're not alone, i have very low self-esteem also tho there's not really any reason for it. like friends get in touch and say why haven't i been in touch, am i ok. that's coz they know when i'm feeling bad i won't contact then incase they're too busy, not in the mood etc...in other words i have no confidence in myself as a friend.
i guess what i'm saying is...it's easier for you if you let people know how you feel, even if you gotta do it by e-mail. real friends understand and accomodate it, do things to help your self-esteem along.
also, i don't know if you have a T or are on meds etc but a good T and the right meds can make a difference also.
take care.