I also want to mention that my first lucid dream started a little bit strange(according to the information I know).
Be extra careful about this: just because we all lucid dream in the same way, doesn't mean the mechanism behind it are exactly the same. Just to give you an example, many people say you can't read small things or even watch time properly in dreams, and many of us done it loads of times. It's also said that if you get too anxious you will wake yourself up, and we have loads of dream reports of highly emotional lucid dreams that lasted ages.

I was preparing 2-3 weeks for lucid dream by DILD method. But there was one girl that I can have only in the dreams so one night I said to my self :"This night she is my in my dream". And this night I entered for first time in lucid dream. But with the time, i stopped liking her so much and now my lucid dreams don't involve her much. Is it possible this to be the problem with my concentration?
There's 2 things you need to consider here:

First, mantras don't necessarily present you with the specific cues. This is, just because you were using the sentence "This night she will be in my dream", doesn't mean it wasn't other factor inducing lucidity: many onironauts use mantras related to their tasks, and the fact that they have an action that needs completion triggers the extra alertness that might induce them a lucid.
Second, dream content reflects many aspects of yourself, and since you're in constant change, it's not uncommon that certain elements become more (or in your case, less) common as time goes. Once you get more skilled in dream control you should be able to summon her without much problem.