You know, this has a lot to do with what playstyle you prefer playing. In my mind, tanky mages are nice to know how to play, but many of their kits do not generally agree with how I feel most confident about playing my early game.
What you should focus on is what kind of playstyle you feel most confident about. For example, are you one of those people that like to chip down an enemy from a distance until you can instagib them? Or are you one of those people that start with "tons of damage" and either try to immediately force your opponent out of lane or get that sweet first blood?
knowing about your playstyle will help you greatly in the road ahead. So, let's say that you don't mind being in the middle of the action, "caught inbetween the crossfire" so to speak. If that's the case, you should pick someone like gragas, ryze, or chogath.
Or, let's say you most prefer getting that "BOOM HEADSHOT" moment off and sniping enemies from a distance with tons of damage. Then your best bet is most likely going to be for someone like xerath (warning: rework upcoming), or Lux.
In conclusion, however, I have always felt like mid can never be a TRUE tank. Sure you can build lots of health,and chogath really naturally supports that in his kit, but you'll never be a roflstompy as a tank like nasus, or yorick. If you more prefer a tanky playstyle, I would actually consider playing more top. In fact, I've seen many cho'gath players go top and still build ap, and still do really well.
As far as playing a champion well, that will come from personal experience, and help from other guides. I would offer more advice on that aspect however I find that many tanky mids are just not suited for me personally. Does that mean I wouldn't learn them at some point? Of course not, I could do so whenever i wanted. But while I too am still getting a grasp of this game, I feel like staying with what I'm most comfortable with is best for me. You should try discovering what you're most comfortable with too, and don't be discouraged if a plan doesn't go the way you hoped. Either you got unlucky, or it won't work, but in the end it's a learning experience either way.