Firstly, even though you and your partner are a couple you should still operate as individuals and be self reliant. Remember that thinking of yourself and having your own life does not make you selfish.
To make your relationship healthy you have to allow each other emotional and physical space. You do this by getting separate interests, friends and activities while still staying close.
You will notice that when you have your own life, your relationship will get more interesting and you'll be yearning to spend more time with each other. In other words your relationship will get better!
It's important not to sacrifice your identity just because you are in a relationship. It's not selfish to have your own life and your own friends. However when you deny the needs of your partner and don't share yourself with them, that is selfishness. This will inevitably destroy the relationship and may even lead to cheating.