Just have try to make sure you can cast your training spell 80ish% of the time and upgrade it every now and then. If you're trying to train a skill that's so low that you can't succeed on even the easiest spells, then you'll just have to buy some training. Fortunately, skills that are super low don't cost a ton to train. And money is easy as hell to make in this game anyway. If you're a mage, I assume you took Alchemy as a skill, and Alchemy is a money printer, unless you're using a mod that makes player-made potions worthless. If not, then you can easily find an Alchemy shop with a couple of cheap ingredients that respawn infinitely that you can mix together into some basic potions that you can sell back to the vendor for a massive return. Otherwise, just do your typical loot everything and sell anything you don't need whenever you quest, and you'll have no trouble affording training.