To a point we are all narcissists and are after our own self interest. I do not believe friends should be jealousy because you are spending more time with your lover, it's natural. It's what people in love do--spend time together, especially at the initial stage when you guys are getting to know each other. Your friends should know this, but that's not usually the case. It's not about who came first but who matters more. I know it's a bitter pill to swallow but no one will pick a lover over a friend, well I speak for myself. This is somebody you're might have a family with...sent the rest of your life in the same space with, that's a lot.
It's a different case when you totally isolate yourself from them,your friends. As long as you try to meet up once in a while, they should appreciate. It's your time in the first place, no one is entitled to it.
However, like you said we need to keep a healthy balance. This can only be possible through conscious effort. Call, text, meet up once in a while if time permits. Let them know they are still part of your life, but they should also know your priorities have change and if they are good friends, they will understand and support you.