Getting a mere +1 in Intelligence at levelup mean you wont be able to chuck out those spells who cost upwards 500 Mana to cast.
I chose Restoration and Destructionas my class magic schools.. bad move since neither one causes INT to rise, and so I have only about 110 Mana at level 17
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I had that same problem. I used Alchemy to get the +5 modifiers to my INT. The poisons came in handy during combat, especially at higher levels. It's a pretty good source of cash too.
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Conjuration is good, too. That's what I've been using to up my INT.