You know, it’s hard to live with a person once life altering things happen to either party. No matter the relationship (friends, lovers, siblings, whatever) co-habitation is one of the hardest things to deal with because we are all so different. So the fact that you two have been friends forever will probably help you in this situation.
If you want to move out from this situation, it is best to do it now before you get too angry and let words get the better of you. What I mean, is you don’t want to get into a huge blow out and then move and ruin your relationship.
You should let your friend know that you feel her and her boyfriend need more privacy, and you also need a bit more freedom in your living arrangement. You can tell her that the two of you are totally cool, and that you are really happy for her, but it’s time that you tried to pursue other things in your life too but you don’t want to jeopardize your friendship. Continue with the fact that you think it’s time to move out on your own because single life is different from couples life no matter how much you want to deny it, and you think that it’s best to come to an arrangement where you would both be good.
Maybe by giving two months notice instead of one if she can’t afford housing. It will give her time to find something or figure out what she wants to do move back in with her folks, etc).
In any case, the best possible thing you could do is move. You don’t want to ruin your relationship, and it’s hard not to get involved with things when they happen in your house. Feeling resentment for the boyfriend always hanging around, or the fights you and your friend get into isn’t going to simmer down any time too soon. So it’s better to let that be, and just move on.
I think if you are honest but not blunt or hurtful, that you can manage to move out without too much fighting over it. In the end, it would be up to her on how she wants to react, but honestly any true friend would see the strain it is causing you and be thankful that you are giving her so much time to figure things out.
Hope this helps you a little, and I hope you and your friend can see that this is just about accommodation, not friendship.
Take care and good luck!