Apart from medical issues, there’s still plenty you can do to conserve your warmth.
Feeling “the cold coming in” is a misnomer. What you are actually feeling is the heat escaping out. The faster the heat escapes, the colder (and more painful) it will feel.
You can read all the tech anywhere on the web, but probably the best gear to buy would be the Mountain/Hiking type gear which isn’t cheap. You can adapt a lot of it for MTB, where equivalents don’t exist. Have a trip down to a good/reputable outdoor shop and see how they can advise you.
Outdoor Kit Reviews & advice