I have a hard time judging the strengths of cards. At first I thought this was great but now I don't think it should even be mythic.

Yeah, this definitely isn't mythic. It's Spell Blast with an extra mill tacked on. Getting milled is normally good, remember, unless the opponent is playing a dedicated mill deck; a reanimator deck would love for you to counter something with this.

You could split it into two parts, making the mill happen anyway, like Overrule:

> Counter target spell unless its controller pays .
> Put the top X cards of that player's library into his or her graveyard.

Even that is more like an uncommon than a mythic. See also Broken Ambitions, Countermand, Induce Paranoia and Psychic Strike, all commons. This (my split version) has potential to mill for 20 if you have enough mana, which I think makes it not common (although Dreadwaters was), but Psychic Drain shows you can mill for X and get a good benefit at uncommon. So I think that's where this should be.

If you want to keep the mill only happening if the spell isn't countered, then this could be common except that Wizards don't generally like putting spells at common.

Mill wasn't the key feature of this card. I wanted to make a counterspell that felt it should be at mythic since this set is supposed to have a focus on instants and sorceries, and counterspells are to me blue's iconic instant thing.

Aha! I agree, making a mythic counterspell makes some sense in an instant-focused set. I had one in my set Sienira's Facets, where the blue-black faction was focused on instants: Cryptic Demand... oh, except it turns out that wasn't mythic: I made Fivefold Braid the mythic instead. Well, Cryptic Demand could have been mythic :)

Note that in printed cards there's Mindbreak Trap. But yes, stapling a Stroke of Genius to a Spell Blast sounds like a fun mythic counterspell. (Reminiscent of Spell Contortion, but, y'know, better.)