Thoroughly reading and digesting your post, coupled with my 30 years of marriage, I'd venture to say, your wife's weight gain may be due to her "self medicating" with food. . . .
Women are extremely perceptive creatures and especially so, when it comes to the man they love. Don't think for a second that she doesn't already know how you feel about her and that you are receiving attention from other women. Her heart must ache terribly, knowing you do not find her desirable and that you think you look 30 and she's old and fat. That possibly your love for her was based simply upon her appearance and not from your soul. I can't even imagine how that must feel. If you truly love your wife, please look past yourself. Let her know that she is and always will be your beautiful girl and work hard on healing the wounds you may have caused her, before you mention, if ever, anything about her weight.