I read somewhere that it's common for people to plateau at a certain skill level, but some people overcome that. My google fu is not working for me, but I think it was to do with changing the approach. I've also heard of changing how you view yourself, imagining you already have the ability, "what would X do?" (Usain Bolt actually peaked and burned out when was younger, regained with a new coach,
less training, and exercises to compensate for a congenital spine defect). i.e. Also, while at a plateau at one specific thing, there are many other things where effort still works (not overgeneralize from that one thing). I'm also plateauing (at discrete mathematics), so I'm interested in the answer here. Many creative people talk about leaving something that they are stuck at, and doing other things (perhaps a short break, perhaps for a long time) - then the answer (sometimes) comes to them, when they see with a new point of view. Talking to people about it can help (even "explaining to a duck").
Of course, maybe there are attributes that we can't change, like height, that seem as insurmountable barrier. While these may be a limitation in themselves, we may be able to work around them (e.g. professional basketball players under 6 foot http://www.toptenz.net/top-10-shortest-nba-players.php). And, well, there are invasive ways to increase height, medically.