I had the same problem, try setting role-playing restrictions for your character (part 1 of my blog has some that I use), and maybe start a journal to flesh him/her out and keep you interested.
The blog also has a Character Building group that can help if you find yourself at a loss for playstyle ideas, and a Challenges group if you just need to take a break from dungeon delving.
No mods?
Let me clarify, don't use overpowered, non-lorefriendly/unrealistic mods. Use mods like Realistic Lighting, some weapon mods are good, roleplaying mods, fixes, etc.
Lol, getting mods is one of the key factors in sticking with a character and Skyrim itself. Try getting mods that will suit 1 character. Like a mod that adds a certain retexture, maybe the throwing knife mod for an assassin.
But anyway, he doesn't even play on PC XD
What's missing when you play one of your characters? You sound like maybe you want to be baddass in ALL of the perk trees. Obvious that's not possible. Do you get bored real quick? Do you feel that certain races are only allowed to do certain quests?
I have only one main character that has done everything and I don't mind playing like that. Although I do have some regret not making a character for each of the questlines, Theives, College, DarkB, Companions, etc. That's only cause I usually play as a "yes man". If I'm asked to help out, I'll go ahead and help them, whether it's not "appropiate" due to role-playing aspects (that may help since I don't really role-play, so it's easier for me to keep one character and become attatched to him). I got too excited when Skyrim came out to never have that problem of leaving Riverwood and start over and over again or make tons of characters. I create one character on how I like to play and roll with it.
10+ characters and not satisfied? hmmm....yeah your best bet is role-playing I guess. Not sure what exact satisfaction you're looking for. If you can't find anything in any of the characters you have, maybe Skyrim doesn't speak to you.
I, too, have many characters (c. 20 still living) - some sneak, some use magick, some dual-wield, etc. etc. I switch based on my mood. I don't see anyting wrong with switching - I am happy with them.
One thing I do to keep my interest is setting goals for a character. One may do MQ, while another concentrates on the mage's guild, one is a storm cloak, while another is an imperial. And goals don't have to be quest related (my first character's goal was to become a master blacksmith - she met that goal, and I retired her). I think the goal helps me both role-play, and keeps me interested in the character, at least until the goals are met.
Try writing a character backstory.
It might help you to care enough about a character to want to keep them long term