How do you reinvent yourself when you're in a relationship?

I (27F) have been in a relationship with the same guy for a little over 3 years. I feel really good with him, and have no desire to break up, but I kind of envy my best friend (27F) who broke up with her ex and was able to reinvent herself.

I don't feel trap in my relationship, but I do feel trap within myself, as I feel like it's very difficult to change and self-improve when you're in a long-term relationship. Any tips to stop being 'his girlfriend' in my head and start being 'my own person' a little bit more?