I need help...

[Tribanite]

Are you over doing it initially. Going hell for leather on the first 40 miles?
slow the pace down slightly and see how you feel.
Lose all the fizzy pop,and drink water. Eat real food and keep a gel for emergencies.

 

[Sittingduck]

Could it be some kind of psycological issue, that you now have an expectation of feeling bad at 35 miles? Just a thought...
RE: Gels - in my opinion there are generally unecessary, for most types of cycling. If you fancy giving them a go, only one way to find out though.