Sounds like your lacking confidence in your own abilities which usually leads to poor strokes and execution which then leads into more doubt and so on. Its a vicious cycle.
I tend to pull myself out of this by saying 'Oh f_ck it!' and ripping a few balls. By ripping a few I help force myself to take full swings through the ball without decelerrating my racquet head. My first few rips sail long but if I persist they start landing in and I get back in the habit of taking proper strokes.
Remember, if your opponents know you lack confidence they will exploit it (especially if they know your new to 3.5) so go out there and demand their respect by nailing a few shots with authority. This will help you gain confidence and possibly put a thought/doubt or two in your opponents mind.
Racquet: Wilson PS97 String: Prince syn gut @55lbs in mains, ISOSPEED Baseline Control 16 @50lbs in crosses