In addition, many people push harder when they are out of the saddle, making it "harder". But you don't have to. Do some drills concentrating on standing and going easier, and then you will detrain the standing = go harder response.
Excellent advice.
Plus, lose some weight. Trust me, I know. Last June I weighed in at 250. today I'm at 210. I makes a HUGH difference. Not only because you're pushing the weight uphill, but the strain on your cardio vascular system is hugh. you're out of breath because you're out of shape. Did you really think you could get on a bike and just start sprinting upihill without consequences?
the standard i've heard is one kilo = one minute. I wouldn't worry about it too much though, just keep riding, get a trainer for the winter months. Keep riding. You will loose weight if you do so, and you will get faster and your fitness will improve. stick with it my friend!! If I can do it, you can too.