Hi Lost Soul, and welcome to this site! I have some info/experience on this topic I'd like to share with you:
I mention this book a lot on here, but I just have to mention it again because it specifically talks about the "sponge" phenomena and how to combat it. The book is called "Children of the Self-Absorbed" and the sponge thing is called "projective identification." I realized recently that I have been carrying around a lot of my nmother's crummy feelings and that they are not necessarily MY feelings - but sometimes it's hard to tell the difference! Therapy helps. Like other people in this thread, I also find that I accept responsibility for other people's feelings... My therapist calls this "enmeshment". For example, my husband gets upset because I won't have sex with him. My natural tendency is to get upset right along with him, and fret, and think he should find someone who satisfies him more, how I can't stand it, how I can't be happy if he's so unhappy, etc. I have to practice letting his problems/feelings be HIS: I practice rolling over and going to sleep! If I'm doing what I truly feel is kind to myself and others, if the others are upset about it, I don't need to get upset along with them! Like lots of things, it takes practice, practice, practice. I'm also doing some "self-reparenting" as suggested in John Bradshaw's book "Homecoming." In particular, I am reading ten "New Rules" every morning, the first of which is that I own my feelings but do not take on others feelings!