Mattress recommendations

Hi! This is a very broad, open-ended question, but I'm hoping you might be able to help. I'm going to start looking for a new mattress for my bed because it does a horrible job at supporting my sacrum tailbone and low back. I get severe pain in those areas and having a bed where I feel like I'm constantly sinking in is not helping. I obviously want a mattress that is firmer and more supportive, but I have no idea what to try, and what to look for, and what to avoid. Is there a particular brand that really works for you, and what materials work best for pain. I currently have a twin bed, and my mattress is probably 15 years old, memory foam with gel, with the mattress topper made from those same materials.
Thanks for your suggestions! Amanda