why won't my bruise go away?

You might have bruised your bone or your muscle. Those deep bruises take months to heal. I had one bad that lasted for five months. It was white in the middle. Anyway, most bruises are only broken blood vessels beneath your top layer of skin. Others can be deeper and take much longer to heal.

the paintball you were hit with could have been frozen and been going over the standard 300 fps. most fields regulate this to prevent such bruising but it still happens. it could just have been a point blank shot. whatever it was you probably had some minor internal damage, more than any typical hit or bruise. I would say that by now you are fine but if something swells up that much you should get it checked out sooner next time. it is not typical.