Good luck on your hair color journey!

The difficulty you have with bleaching is one issue, the type of color you've been using is a separate issue.

You don't mention what your natural color is, but if it's dark brown to black, you might have to bleach your hair several times to make it light enough to show a pale color, and/or porous enough to hold it.

Developer dyes: Imagine your hair shaft is like a clear straw filled with brown liquid. You need to dilute or strip the color out before you can replace it with another color. Whatever color the "liquid" (your hair) is, add the color you want to that color. If it's orangey, then silver dye + orange = dark ashy blonde, if it's yellow, silver + yellow = lighter ash blonde, white + silver = silver. Bleaching strips the color out from inside the 'straw,' developer dyes replace it, putting molecules of color inside the hair shaft. Toners are for color correcting pale blondes only. Bleach too much and the color will only stain the outside, it won't get into the hair shaft and will fade faster.

Temporary dyes: Imagine again that your hair is a straw, and you dipped it into a bowl of translucent food coloring. The color will mainly stain the outside of the straw, whatever color is inside the straw will show through. The lighter you bleach, the more true the color, and the paler colors will show up as expected. They typically fade much faster because there is not a chemical process forcing color into the shaft.

It's not clear what kind of hair color you are talking about, i.e.: temporary (no developer, like manic panic, punky color, etc.) Demi or semi permanent, toner or permanent dye (all used with developer.)?

I would advise against mixing hair colors until you have been dying your hair for a while and have read more about it. Temporary colors cannot be mixed with hair dye that uses developer, and I've personally had problems mixing different brands of temp color, it's often hard to tell what the "base" color is, especially with silvers and grays, not to mention the formulas of the dyes can be incompatible (the carrier for the color.) Even with temp colors, you can really get some bad results and waste a ton of money.

(I've used 4 kinds of silver/gray temp color, one was blue based, one purple based, one was an almost true colorless gray, and one was brown! They all look virtually the same out of the tube, but after fading you could see the base colors clearly.)

Regardless of the type of color you used, Silver, gray and pale pastel colors are incredibly hard to maintain, and different brands of hair color have their own unique longevity. You should not compare the longevity of a bright blue or hot pink to a pale gray, these are very different products. Gray and silver temp colors do start to fade after one wash, especially if you use a harsh shampoo, like a sulfur heavy dandruff shampoo. Permanent colors and toners last longer but are harder to mix a custom color, I would recommend getting it done at a salon until you learn more about the products.

I hope that helped! My advice to anyone is, if you want to get into DIY hair coloring, please read everything you can and keep asking questions before starting to experiment on your beautiful hair!