Stress

Obviously it is certain you are sufferig from stress. But it is not certain you can be helped because it may be impossible given the conditions. I will try to explain. How many lawyers are in your firm at your level? If there are do they also have the same workload? How do they handle all the files? If all the work is funneled to you, you are faced with an impossible task and you should consider leaving if your boss can not offer you any relief. If there are others at your level and with similar workloads find out how they handle it. If you find you are faced with an impossible situation you should leave. Your health and future are more important than anything else. I don't think your best friend realizes your stress because you haven't told her. You must speak up not everything that hurts you is obvious to someone else.

K,   Rosie  gives good advice.  Does your boss interrupt with business or more social issues? If business, that is more acceptable, but also more problematic.  Have you considered a little, but noticeable, placard sign for your desk "Out to Lunch...back at work by ____" with a place to write the time...of put in on it if is always the same time.  The boss will then have no excuse of not knowing and may get the hint and come back at the implied time.  Are you in an office or cubicle or out in the open at a "table"?  If you have a wall or something to face curing lunch, turn your back to the approach that the placard is "guarding"....or take your lunch and go outside or to a nice quiet secluded part of the building where the boss will not necessarily be able to find you easily....it is your official and sanctioned "lunch break", right?   When she asks you to do things not in "the job scope" (your job description or the company's defined and offered services?), you might very politely and casually as "How do you want me to bill the time?" unless that is already clearly defined.  How much older, or younger, is she than you? Is she an owner, partner, or other officer of the company?

Just be honest with your friend. Persuade her. Guide her conversation if possible with questions or comments.  If she does not respect your feelings as expressed, then tactfully verbally cut her off when she starts to ramble.."What exactly is the message your are trying so eloquently to convey...?"  As an attorney, you need to practice your "wordsmithing" skills or you will not go far in the profession if you want to do more than office work and have ambitions for court room roles or authoring of documents.  

K,  Given your comments in the above response to Rosie, please consider the possibility that your boss greatly values your participation and reliability so takes "special" tasks to you.  As such, she might greatly regret losing you if you leave. Consider inviting her to join you for lunch to discuss something that has been bothering you, hang your "Out to Lunch - Do Not Disturb" sign on the door (explaining to her that it is so people will not DISRESPECTFULLY DISTURB YOUR LUNCH time with her) :),  close your office door, and after visiting and while eating, casually express your thoughts and feelings to her about how much her interruptions during your one hour of lunch and rest time disturbs you and that you feel the only alternative she is leaving you, unless she can respect you more and not interrupt your lunch period, is to leave...clearly stating that you do not want to.   I would have this lunch session with her after you identify a possible relocation career position elsewhere, so that if she ignores your feelings or tries to retaliate in response to a perceived insubordination, you can actually depart with minimal damage to you and your family (if you have one dependent on you).