You can't always tell when you're ovulating. You can count days, take your temperature (it raises a little when you ovulate), monitor your cervical mucous, and pay attention to how you feel. You may have a little pain, soreness, or discomfort on one side of your abdomen which indicates that side's ovary is producing an egg. You can also use an ovulation test kit that you can buy at any pharmacy.
As for the fibroids, they can cause fertility problems, but not necessarily. You should consult with your doctor about this when you are ready to conceive again and if you have trouble.
I think your main concern should be how you feel about your life right now. You say you feel like you are destined to be alone, but that's not true. You have three children. That's not alone. Yes, you want a romantic partner in your life, but you need to make sure that things are right with you, emotionally, mentally, spiritually so you will attract the right kind of guy that you can spend the rest of your life with. Don't settle just so you won't be alone--being with the wrong person is the most alone you can be. And if you don't find the right guy, being single doesn't have to be horrible. Focus on yourself and other things that make you happy, whether it be family, work, taking classes, hobbies, reading, or whatever. A partner doesn't make you a whole person. Only you can do that for yourself.
I wish you all the best.