When you say your current plan allows you to almost keep up, does that mean you've gone out with the A group and have gotten dropped? You may already have the fitness, but not the group riding skills. There's a minimum fitness "buy-in" of course, but fitness isn't the entire equation. There are old racers on my local group rides who might show up 20 pounds heavy and 50 watts light, but they never ever get dropped because they know when to hide and when to burn a match. Is there a particular place on the route where you usually get dropped, like say a particular hill or a prevailing crosswind?