2) STOP PLAYING. Go at least one day without Skyrim.
Yeah, I gotta suggest more than that. I'd say go about a
week without playing. I hate to suggest that somebody not play a game when the suggestion comes on that game's official forums, but you're obsessing a bit. I'm not being sarcastic when I say this, but perhaps you've got a touch of mild OCD, depending upon why you have "restartitis". Are you thinking that you've done something
wrong with the character by level 40 - as in, "this isn't how I wanted the characer to turn out"? That seems a little neat-freak-ish to me; nothing in life is perfect, including an RPG character. There might be the odd wasted or mistaken perk choice here or there, but so what? Don't let that bother you.
On the other hand, perhaps you're just
getting tired of any given character by level 40, and you're feeling the itch to try something different. That's perfectly normal, and frankly I think it would be odd if you didn't. I really don't believe in trying to do all or even most of the game with a single character. Finish the main quest with each, sure, but not all characters would do all quests. My current character is a level 43 Nord Warrior-type. One-handed, block, heavy armor and smithing are all being concentrated on, with a couple perks in archery and a single perk in Restoration just to make my healing more effective. So, a warrior with a little practical knowledge of the magic he's most likely to need - healing. I'm gonna finish the Companions quests, then probably head into completing the main quest. Haven't touched the Civil War with this guy, not doing
any Daedric quests, and of course no Dark Brotherhood or Thieves' Guild. Was doing a quest about a wizard's amulet (won't give any spoilers) until I found out to finish it I'd have to go do a quest that my character would never do.
So while he's explored all the major and minor cities, I've probably done most of the quests that I wanna do with this character.
Next up is gonna be a sort of warrior-necromancer-fire mage-magic item-making dude. Sauron Jr., I keep calling him. One-handed, heavy armor, smithing, enchanting, Destruction (fire only) and Necromantic Summoning. I think the term I've heard is "Death Knight", although I think maybe that originated with Warcraft 3. Possibly a little more spread out than the first Viking, but no archery, sneaking, other magic schools or anything else to dilute the concept any. Oh, and Daedric armor as soon as I can. As some have said, Daedric looks like it belongs on a heavy metal album cover, or possibly the cover of Heavy Metal magazine, circa the late seventies, and it would make no sense for my Viking, who's rocking the Steel Plate armor and an Ancient Nord Helmet, but it'll be perfect for ol' Sauron Jr. (I really gotta think of a real name.)
And after the second character, I'll probably play some sort of stealth-oriented character, possibly an illusionist-thief, since I have yet to play that sort of character in an RPG, but I don't have any idea about him beyond that.
So try that: Don't work to stick with a given character for too much of the game. You want to experience all of the game, but you don't need to experience it all in one playthrough, and playing part of the game with one character, and part of the game with another will keep it fresh.