Ive always loved Maths but for numerous reasons I bounced along in general/non-profit admin/management. After maternity leave I have found myself at the bottom again and seriously thinking how can I get back on track. Im 37, I graduated in 2000 with a UK BSc in Math 2:1. Anyone I chat to (non-mathematicians) always say, "it will come back to you" " go for it" etc but what do they know? What do those working in Math think? I especially was drawn to discrete maths, logic and computability. Where are the doors?
To do mathematics professionally you should enroll to a graduate school. I know at least two similar cases where people enrolled to a graduate school after a long gap and became successful professional mathematicians (one of them in UK).
Of course there is also a way of self-education. But the choice depends on many other factors, for example whether you need to support yourself and/or your family etc.
I think this depends very much on whether you want / need to earn money with doing mathematics or not. If you don't need to earn money with it, just go ahead and enroll to a graduate school. Though if you do need to earn money with it, you may find this pretty difficult if you start now with just a BSc.
If I'm in your shoes, then I would enroll to a computer science graduate program (first MSc and then PhD if everything goes well). Every computer science department offers courses in the areas that you're interested in. Then, if for some reason you get bored doing research, with your background, you should be able to find a job in industry.
I suggest that you start by finding someone who does discrete mathematics or something similar on the website of a nearby university. Alternatively, you could find an MSc admissions tutor. With a modest amount of luck, you will find someone friendly and helpful who will be willing to spend half an hour talking through the options with you. They will be able to ask mathematical questions to gain some sense of how much you remember, which will be required for any realistic advice.