You're doing it wrong. Not that I can blame you - unless you take the time to work it out, or find someone else to tell you, it's easy to have trouble finding resources.
The key is knowing where they are, and the best way to get them. Diamonds are only near the bottom of the map, and the best way to find them is to dig long tunnels just above the lava layer. You take a few iron tools and some basic equipment with you, dig a long tunnel until you've broken half of your iron pickaxes, then dig a parallel tunnel back to where you started. You'll pick up enough iron to replace all the tools you broke and then some, a lot of coal, and a reasonable amount of rarer materials.
It's still time consuming, but tends to be much more effective than wandering around caves or digging randomly.
Or you could cheat, and use some mod that helps you find ore. There were mods that add xray vision, or auto maps that can show the position of hidden ores.
Or just use creative mode, if you just want to build stuff.
The key is knowing where they are, and the best way to get them. Diamonds are only near the bottom of the map, and the best way to find them is to dig long tunnels just above the lava layer. You take a few iron tools and some basic equipment with you, dig a long tunnel until you've broken half of your iron pickaxes, then dig a parallel tunnel back to where you started. You'll pick up enough iron to replace all the tools you broke and then some, a lot of coal, and a reasonable amount of rarer materials.
It's still time consuming, but tends to be much more effective than wandering around caves or digging randomly.
Or you could cheat, and use some mod that helps you find ore. There were mods that add xray vision, or auto maps that can show the position of hidden ores.
Or just use creative mode, if you just want to build stuff.