i don't water that often cause the soil always seems wet
This indicates to me that the soil is too dense, finely textured, organic.
i repotted this year with a compost thats it cause the guys at the shop told me compost should suit it fine.
When I think of compost I see a 100% organic material that is great in a garden but horrible in a pot. In the UK I believe they use the term differently.
my mum thinks there is too much sun outside is that true
That may well be but I think you have other problems now.
what about the white insects i see under the flaky bark?
This is worrisome and short of spraying with something the only suggestion I have is to let the tree dry more. It is my understanding that this species can react badly to pesticides anyway.
You could try re-potting it into a correct medium but the stress of another move could prove fatal to a tree already in distress. Besides until you have a better understanding of bonsai mediums there is no point in re-potting into something new that is still inappropriate.
It may be too late for this particular plant. If you have ever taken cuttings you could try to salvage the plant that way. If it dies accept it and move on but do not give up on bonsai. We all kill trees especially early on. Take this as a hint to do more research next time, preferably before you buy.