My two-year-old daughter plays let's-pretend, but she also has a more immersed mode of roleplay where she says, for instance, "I'm really a rabbit. Not pretend!" It's her way of controlling the "immersion dial" of her game.
In D&D, adults control their "immersion dial" by adding the two mainstays of adult living: bookkeeping and fear. As Agent Smith says, "Human beings define their reality through misery and suffering. The perfect world is a dream that your primitive cerebrum keeps trying to wake up from."
This misery-and-suffering dial needs a couple of settings. Most gamers require rules to make their fantasy worlds seem real, but some don't like their role-playing investment wasted by arbitrary character death. More jaded RPGers need increasing risk of death to reach the same imaginative high. From session to session I vary from one pole to another.
Here's a way to crank up the dial on a session-by-session basis: make a Hardcore Mode character.
One Hardcore character at a time. You can have as many casual D&D characters as you want, but you may only have one Hardcore character at a time: that's your "official" character until it is permanently retired (often because the character died and can't afford a Raise Dead). Once a Hardcore character is retired, it must never be played again.
Decide before character generation. You can't look at stat rolls of all 18s and decide "This is my hardcore character." You declare Hardcore Mode before character generation, and you live with whatever you get. However, the method of generation is up to you and the DM: 4d6 drop the lowest, 3d6 in order, point buy, DCC's multi-character funnel, whatever.
Start at level 1. No matter how bored you are with level 1 of D&D, or what level the other characters in the game are, a Hardcore Mode character must start at level 1 and earn their way through every level. If your 15th-level Hardcore character dies, you can either create a casual 15th-level character or try to survive as a Hardcore 1st-level character in a high-level campaign. The way that the character earns XP is up to the DM, of course: XP for monsters killed, XP for GP, "it's been a couple of sessions so you all level" are all fine.
Multiple DMs and editions are OK. You can import your Hardcore Mode character into the game of any DM who will allow it, jump willy-nilly from one campaign setting to another, and convert from any D&D edition to any other. You can rebuild the character according to local rules, but must start with your most recent six attribute scores and progress towards the next level (for instance, 1/3 of the way to level 5). Thus, a 1e character could be rebuilt in a 4e game, but must have the same Constitution: its HP would be recalculated by 4e rules. A character who leveled up to 4 in a 5e game would get a stat boost, which they would get to keep on return to an OD&D game. Possessions from another game may be temporarily re-interpreted or ignored by the local DM. Hardcore Mode doesn't imply any particular level of lethality or treasure stinginess.
Here's another fun option, but I won't hold you to it: when you say "let me tell you about my character" (and you will), you can only talk about your Hardcore Mode character. By the way, I consider my "hardcore" character to be Roger de Coverley, my 6th level OD&D thief from Mike Mornard's campaign.