I think the guy who suggested a skipper is right on. Your design is all thrust and no specific impulse (efficiency). You're running out of fuel part way up. As you get higher, you won't need as much thrust because you are shedding weight, the air resistance is dropping, and you are also coasting off the momentum of previous stages. Skippers are a great choice. They still have a ton of thrust and use much less fuel then the mainsails. I've also had some success with using a few aerospikes in a stage, as they have great specific impulse (though poor thrust). Also, terminal velocity is an issue. Your thrust is giving you diminishing returns once you hit it. You may find that all of those Mainsails aren't actually helping much more than four or so would. I'm far from an expert, but I would jam some extra solid boosters on there to get that early thrust you need (best thrust to mass ratio in the game), cut down on the Mainsails, and replace them with Skippers.