Firstly, I will second all the comments about concentrating your weekly exercise in to one 3 hour beasting session on a Sunday. It's not an ideal way to operate and is one of the main contributary factors to your problems.
Anyway, the reason that you're feeling shattered is that you've completely run out of energy. That sounds kind of obvious, but let me expand a little.
You are doing a fairly long endurance effort and you're not fueling your body sufficiently either during the ride or afterwards. You admit that you're out of shape and you're going pretty hard. This means that your body is going to preferentially consume glycogen and readily available carbs for its fuel. Once you've blown through that, which you're liable to have done over 3 hours, you're going to be wasted and your body is screaming out for fuel. You're then denying your body the fuel it wants in a misplaced attempt to accelerate fat loss.
Without wishing to second-guess your lifestyle, I would suggest aiming to restrict calorie intake during the week and concentrate on taking them onboard whilst riding.
Recovery starts before you've even gone out the door. A decent meal with a mix of simple and complex carbs about 3 hours before exercise will help fuel your ride. Keep eating and drinking during your ride and when you get in after a hard effort, make an effort to recover properly. That means taking on carbs and a little protein. This should be in a ratio of 3:1 (which is the constituent make up of all recovery shakes). Aim for things like pasta, potatoes, vegetables and a little lean meat, along with some fruit juice.
Taking a protein shake only will help a little for your muscles to repair, but it won't hit your main problem of restoring your glycogen stores and this is exacerbating your exhausted feeling.
In addition to re-fueling, one of the best ways of recovering is good quality sleep. Try and get to bed early the night after a ride and avoid excessive alcohol.
rob
