It’ll be hard, but what you said here is what you tell her. It’s about you needing to be better and really process the last relationship and what’s currently happening in your life. Compliment sandwich it. Let her know she’s great and remind her of some nice moments, explain the situation and why it’s important to you, listen to what she says, and thank her for it. But take your time to think over whatever she tells you. Emotions will make it hard to do, but be firm to her that you need this time and that she has been nothing but great. Give yourself a couple days to cool off after you’ve made it official so you can recover from it. It’s basically a breakup with strings attached, so be prepared for sadness.