They always describe me as shy, sad, emotionless or serious.
I'm a mix of apathy and introversion. I suspect my apathy is caused by bitterness.
I have a hard time joining conversations because I have nothing to add to them.
I could be hanging out with other people for hours and not say a single word the whole time while they're going off with their small talk.
What do?
I suspect that you are boring.
I also suspect that you may be overcompensating social anxiety by acting like you are desinterested, too good, too edgy, too intelligent, too special, too non conformist etc.
The most important part is break with your habits and make new experiences. If usually you don't say much, try to chip in, start a new topic, open up etc. Obviously you suffer from the way you are acting, so change it. It will feel awkward and very stressful in the beginning but it gets easier.