I'd second the advice about breathing in through the nose and out through the mouth. I have my very first sportive this weekend, 35 miles over a hilly route and have joined a women's cycle club to learn the best way to tackle the 2700ft of elevation I'll face. I was told about the breathing thing, also sit well back in your saddle, it puts more power through your legs. Relax, use your mantra and a low gear and spin up the hill in your own sweet time. It works.... it's improved my climbing and I'm finding the more I do it the faster I get, almost without trying. I live in a hilly area and its great to face a hill and not be thinking ' OMG I'll never get up that'
Although there are a few steep 15-25% stinkers that await the time when I'm even stronger and can tackle them with confidence. Good luck!