Not so sure that past drug use has anything to do with it, I speak from experience as I abused heroin and all sorts of hallucinogens for years. You need to understand that the mental consiousness is not like the other sensory consiousness, so don't expect to get a clear and stable image right off. If you are doing deity yoga, there is much more to the process than just creating a visualization. You are using the imaginative abilities of the mind to create an overall "feeling" of being the deity. This of course needs to be supported by the transmission of empowerment. Alexander Berzin gives some great advice on visualization on his website www.berzinarchives.com
. If you are using visualization for shamatha, it can be usefull to start by using an object for support, such as a picture of a Tibetan or Sanskrit syllable. It is just something that takes time so don't get frustrated. I had a really hard time learning to visualize Tibetan syllables facing outward, I was so used to seeing them in a certain way. It is still difficult for me after years, so don't get too hard on yourself and certaintly do not doubt you ability, just keep trying!
Have you read anything by Khenchen Thrangu Rinpoche? He is a master scholar and meditator of the Karma Kagyu tradition. I believe his teachings on meditation and the mind would be useful to any meditator, even if they do not practice in the Tibetan tradition. His website is www.rinpoche.com
and there are a ton of profound teachings. Check it out if you haven't already. Hope this helps