How can a person know that God has accepted from him, especially if he feels frustrated and that prayers are not answered, and fears that his acts of worship are scattered dust, and feels oppressed and without a helper?

When fearing that good deeds and supplications might not be accepted, one must have a good opinion of Allah, for Allah does not let the reward of those who do good go to waste, and He answers supplications. It is stated in the Hadith (definition) Qudsi: "I am as My servant thinks of Me." Allah has promised to accept the work of one who perfects his work, as He said: "Indeed, We will not allow to be lost the reward of any who did well in deeds." For a deed to be perfected, two conditions must be met: it must be sincerely for the sake of Allah, and it must conform to the Sharia (definition) of Allah. Supplicating for the acceptance of deeds is from the guidance of the prophets. It is recommended to consistently recite the prescribed remembrances and protective prayers such as "the Mu'awwidhat (Surahs Al-Falaq and An-Nas)" and "Hasbiyallahu la ilaha illa Hu (Allah is sufficient for me; there is no god but He)" seven times in the morning and evening, and to repel thoughts of feeling oppressed and seek refuge in Allah from Satan.