Join the gym and lose your weight. Do not or try not to look desperate, this will just confirm leaving you is the right thing to do, she will feel she can do better than you.
If you are going to do date nights, then just do it, dont ask, be assertive take the bull by the f***ing horns and show her your a fun loving INDIVIDUAL! not part of couple.
IMO she has found someone else, it has all the signs. She has lost weight, so it maybe someone whos been in the background for some time and has suddenly noticed a 'new chick'. Its gonna hurt but start to look around, emails, texts.....go into work or send her flowers ( note should read thanks for last night ;-) ) see her reaction.
Good luck, its a hard path your gonna have to walk but we have nearly all done it.