How do I overcome being jealous of my boyfriends mother?

Try to think of his close relationship with his mother in a good way. It means that they have a positive family structure which can transfer down to your own family if you have one together in the future. 

Despite your boyfriend having a good relationship with his mother, it will never be equal to the relationship that you have with him. A love for a mother is a very different love than one with a girlfriend or wife. There are certain things that he may look towards his mother for and certain things that he may look toward you for. 

So, instead of comparing your relationship with him to the one he has with his mother, try viewing them as two separate entities. She can not replace you, and you cannot replace her. However, all together, you can have a positive, healthy family dynamic.