I kinda had this problem until I did a bit of gear management.
Yes I got my enchanting and alchemy to lvl 100, but I didn't do that one exploit that makes your stats go through the roof. All I have is fortify destruction and restoration +100 across all my gear. Even before I got to that point, I was just using spells that did not cost an insane amount of mana and turning every fight I got into, into a game of cat and mouse.
Here are three strategies that should help you:
1. Change your gear: I imagine you either have a lot of +magicka or +magicka regeneration. These both suck since once you get to the Expert lvl spells, +50 magicka is completely useless for 250mp spells. Magicka regeneration also doesn't work like it does in most games. Out of combat, your regen is something like 3% of your total magicka a second. In combat, it feels like half if not one third of that. So you will be running around in circles waiting for your mana to regen. Do yourself a favor and either buy or enchant your gear so you have as much fortify destruction as you can. Also, make sure you get relevant perks. If you have the Adept perk, use Adept spells more than Expert. If you have Expert, then use them and so on. Each perk in the tree only affects the lvl of spells that it says it does. You'd think that the Expert perk decreases the cost of all destruction spells by half, but it doesn't. The only one that affects all lvls of spells is the one you get at lvl 100, and I think it is a 15% decrease across the board. That coupled with the fact you are lvl 100 should decrease all spells to a pretty low amount.
2. Change your play style: get elevation on the enemy(s), funnel them into a hallway, double cast runes on the walls, use a summon to distract and trap them, use aoe spells, etc. I stopped using Fireball and it's Frost counterpart once I got Incinerate, but until you do, the fact that they do AOE and you can throw them practically anywhere and it'll hit the enemy, is awesome. It is a shame that the Master lvl spells are shit...
3. Become a drug addict: If neither of these fit you, your last option is buying lots and lots of potions. Run out of mana? Down a bunch of pots. Run out of hp? Down a bunch of pots. Etc.
I am interested in what your talent trees look like and what your magicka/hp/stam is like.
I got my mage to 200 hp and stam then burned every point I've had since then on magicka. I think I have 762 magicka? I only use it for my mage armour and the occasional summon though since I have fortify destruction and restoration, lolz.
Related: Conjuration sucks balls once you get to lvl 40ish. Pets don't do enough damage, don't aggro at all, die way too fast, and you're always summoning them again and again... It is especially irritating when a Dragon Priest steals them from you and uses them against you >=/ Illusion is under-rated and epic win. Especially if you are a Vampire. Fearing and Calming everything in sight is hilarious devious. Oh, and the 3 perks you get after the +damage perks in destruction, are useless. The "enemies run away when low hp", "are paralyzed when low hp" and "may disintegrate when low hp" are three perks I wish I used elsewhere =(