OP, I'll be honest with you. You confessing your love for her will already be affecting her relationship. You're her ex-boyfriend. She clearly still cares for you since you're still friends, but that would have been hard enough for her new partner without now knowing that you're still in love with her. If she still loves you, then she will say that. But you have to be prepared that she may not.

Your confession is likely going to change things and you have to be ready for that being a positive change or a negative one. My advice would be to put a bit of distance between you both while you take some time to heal. Being around her constantly, texting and calling her daily, seeing her with someone else, all while you want to be with her... that's really difficult to move on from. Give yourself some time to recover from the breakup because you clearly haven't. Wishing you all the best, OP.

You can still talk to her and respect that she has a relationship already.

>i don't want to affect her relationship

Very noble, and very easy. Essentially there is just one thing you need to avoid in order to do this; make sure you don't tell her you love her.