What should one think about during prayer? Can I for example during one of my prayers only be thankfull to Allah, and think about my love for him, and in the next prayer maybe concenrate on my sins, and regret those during the whole prayer, and alternate in such a way? Or should there be something specific to have in mind whilst praying?

Yes what you mentioned is fine

Please read Imam Khomeini’s secrets of prayers book for more