I buy games I know I won't beat.

I don't think this is a bad habit.  It is just being smart.  I do it too.

I have the habit of spamming buttons while playing hack & slash games even though I know it won't work if it's on the highest difficulty

Yes, this is a problem for me, though I don't think it is necessary to beat every game I buy in order to get my money's worth.  

For some unknown reason I try not to use the best items the game gives you :P
Ethers or elixirs on rpgs for example.

I'm with you.  Always thinking I'll need it later or if I get stuck.  Hell I do it with shooters like Doom that let you have great weapons and save their ammo.  I'm always walking around with BFG rounds and gattling gun fully loaded.

Guilty of reloading after one shot, getting useless collectibles, respawning from a save point every time I get detected in stealthing, looting everything, hoarding, and long on vs off periods with games before finishing them ...

I used to be a lot more precautious but my main problems now are over committing and taking unnecessary risks. Sometimes I just want to see how far I can get with the bare minimum of supplies, such as my attempt in Super Metroid of getting as far as I can without essential items.

I buy games I know I won't play.

'Taking a break' 2/3rds through a game, then not touching it for months/years. By the time I finally come back, I might as well restart from the beginning.