Is what the questioner does correct by saying, "O Allah, to You be all praise for every blessing," in order to avoid doubts about being thankful for blessings, and what do you advise her to do to increase her faith and certainty in Allah?

One should not give in to obsessive thoughts. Thank Allah generally for His blessings and ask Him to grant you the ability to thank Him. Persist in reciting the prophetic supplication: "O Allah, help me to remember You, to thank You, and to worship You in the best manner." One should also focus on actions that aid in showing gratitude, such as contentment and being keen on the comprehensive supplications for thanking blessings, like saying: "O Allah, whatever blessing has come to me in the morning is from You alone, You have no partner. To You belongs all praise and to You belongs all thanks" in the morning and evening. The greatest way for a servant to thank his Lord is by performing acts of worship, especially the obligatory ones. If the obsessive thoughts persist, it is advisable to consult a psychologist. To increase faith, it is recommended to read the Quran and books of Sunnah (definition), such as "Riyadh as-Salihin" and "Al-Bahr ar-Ra'iq," as well as books about faith.