Nostalrius Begins - Quality wow vanilla realm (1.12)

This happened to me in vanilla, and every expansion since (I stopped at Cata)
I am in absolute love with the game until I get to mid 30's I play until by eyes burn, I've played wearing sunglasses to make it hurt less because I didn't want to stop playing..

then by level 40 I'm completely burned out.
I get sick of grinding, bgs, I lose the effort required to find groups for dungeons.
This is usually when I roll an alt, and get them to around level 20, get bored.
Roll a new alt to level 5
Then quit for 6 months to a year.

Played vanilla through cata and never once had a single toon at max level for the most current version of the game.

It blows my mind when I see people with multiple maxed toons at any stage of the game. How do you do it? How do you keep pushing forward through the mid to late levels when everything feels sooo slow?

I think a solution worth trying would be to take a break for a while.
In fact, make a specific date for you to come back on. If you find yourself wanting to play before that date, it's working! Human nature is to want what you don't have, after all.

I hear you. Personally for me it's about getting caught up in the race to end-game - and those midlevels really are a dry spot. Since you are asking for a solution to stay in-game (and on char), I wont suggest you to take a break; but maybe mix it up a bit? Do something totally out of your comfort zone (world PvP or RP?), or focus on professions or gold making. They are each a mini game in their own right. Pets are also a thing in Vanilla, though less so than retail.

Having goals would help too. Otherwise, why beat yourself and waste time on the game?

Take breaks, there's no point to just sit and grind levels nonstop, because doing so ofc will burn you out and make game look boring.

do some lowlevel pvp.

you can gain up to rank 8 in the level 40-49 bracket and then you can buy some nice items for level 60.