Few tips from a fellow insecure colorer:
https://www.amazon.com/Coloring-Crafts-Coloristas-Ready-Color/dp/1497200873
Invest in some alcohol markers with brush tips. They are super vibrant, easy to blend and layer, easy to correct mistakes with and they're a dream to use. I recommend Ohuhu brush & fine tips for affordable high quality markers with a good color range. (216 colors max for this line)
Get a big mandala book, pick 4-7 random colors, forget about if they go well together or not. Then color the whole thing consistently sticking to the same colors for the same parts. So for example if you color a certain swirl red, use the same red every time you see the same swirl.
Also forget about using multiple colors or creating swirls or dots or whatever extra decorations that is not already in the pattern. Just color the sections that are already printed.
It'll look like crap for a while but when it's finished something magical happens. You'll discover that even if you picked colors you would normally hate together, once the mandala is finished it still looks great. The symmetry and the repeating few colors makes it very pleasing to the eye.
For this reason, I recommend sticking to repeating patterns instead of freehand books where you need to use 20 different colors.
And don't use too many colors, theore colors you use, the harder it becomes to make it look good together rather than a mess.