Oh bless you. I can 100% understand your cold feet, that's perfectly normal if this is your first holiday on your own. But remember there are millions of women out there who are single by choice, widowed, divorced and separated and many of them travel solo.
They all had to take the first step and I'm sure they were terrified the first time also. Take that first step. I think it will be a milestone for you and will increase your confidence by miles. It's not easy, you are scared, feel lonely, wondering if you will "stick out" as a solo and so on. You won't, there will be hundreds like you on board!
Take it in baby steps. Get to the ship with plenty of time to spare so you're not stressed out. Go to your cabin, read all the information, go to a general meeting/group and suss out your sailing companions. Share a table, if it doesn't work out and you want to bolt, you can go back to your room and get room service.
It's all very fine us telling you that you'll be fine, it's how YOU feel, however if you do go you will be so proud of yourself and I guarantee that you will enjoy it as long as you do only what YOU want to do, and avoid any Hi de Hi organiser passengers (unless you want to!).
FWIW I travel solo a good bit. That's because I enjoy archaeology and Greek and Roman history but the other half does not. It was a challenge the first time, but now I relish it. I'm 68 BTW. Best of luck to you. I think going solo is far preferable than hooking up with someone you don't know from Adam anyway. She might turn out to be a bully, a charmer, a thief, a manipulator and so on. She might be fine, but you won't know. I wish you well.