it may not be the saddle, but your fitness. I've heard that your legs support much of the weight (together with your arms and the saddle) when you're on a bike. from what you've described, it could be that you're trying to do too much too soon, and your legs are getting fatigued (trying to do what you did last summer) when they havnt readjusted. Weaker legs mean that less weight will be held up by them, so you'll sit "heavier" on the saddle.