Well, guess what... I always end up painting something different from my initial idea... the important thing is that I keep working on it until eventually I'd like it..

Also, I suggest you do lots of studies.. like literally copy photos of nature, animals, etc and learn from their structures, colors, etc.. this way, you would store a visual library in your brain.. the more you store.. the easier for you to clearly paint your visions... and eventually, you'd do more realistic paintings... no other way but lotsa practice~