Don’t think he will ever grow up!

Should I end a relationship if I think there is no potential for it to evolve in any way? I only ask because I am madly in love with my partner and can’t imagine life without him. His inability to grow as a person, and an adult, are what make me feel a sense of despair. This is someone I enjoy being with, but also someone I know is incapable of being mature even after we grow older. I am thinking of my future while making a decision about the present, and part of me wonders if that is a sensible thing to do. Should I just enjoy this while it lasts until it can’t progress anymore? Am I just causing myself unnecessary pain?
This depends upon your ability to separate fact from fiction, and how you prioritise this relationship against other aspects of your life. If you know this isn’t going to work out, that self-awareness comes with unavoidable pain. Choosing to stay on in a relationship that isn’t going anywhere is fine only if it meets your current needs. There’s nothing wrong with planning for the future either, because ending this now is effectively an act of self-preservation. If you love this person, ending this will hurt you irrespective of when you choose to do it. Ask yourself if putting in the work is worth it, given that you don’t see a long-term commitment with this person.

My boyfriend refuses to stop talking to a girl he used to have a crush on. He says there’s nothing between them and that I am being insecure. Is he right?
This isn’t about him being right as much as it is about wilfully denying how you feel. He is in a relationship with you, and the onus of making you feel secure is an integral part of this. If he can’t do that, and is asking you to deny your feelings, this is a red flag and signifies a lack of mutual respect. Call him out on it.