Doubts - that beset the mind concerning worship or matters of belief, even the essence of Allaah - are from Shaytaan. When the Prophet's Companions complained to him about doubts that sometimes entered their hearts, doubts they thought too wicked to speak about, the Prophet Salallaahu alayhi wasallam) told them that, that was because of their faith, meaning its purity.
That is because Shaytaan whispers doubts about faith only in those hearts that are free from any doubt, hoping that those hearts will obey him. Meanwhile, Shaytaan does not need to whisper into a heart that is already beset with doubts because he has already successfully completed his work on that heart.
I advise you to seek refuge in Allaah Almighty from Shaytaan; ignore his whispers, regardless whether they come to you during prayer, during ablution, or at any other time. Although these doubts indicate a pure faith, if they continue, they may also indicate weak willpower.
Try your utmost to ignore his whisperings. When you bring an item home from the store, do you doubt that it is in your hands? The answer is obviously no. This is because the Shaytaan does not whisper to man in these kinds of affairs; he only whispers to him about his worship so that he can ruin it for him. Also, when you have completed an act of worship, don't have doubts about whether it was performed correctly, unless you are sure that there was some defect.
You mentioned your doubts about food; the general ruling on food is that it is permissible, so your doubts are not valid. A jewish women gave the Prophet Salallaahu alayhi wasallam some lamb to eat at khaybar, and he ate it. A jewish man invited him to eat and presented him with barely bread and some other food and he ate it.
In Sahih Al-Bukhari it is reported that some Muslims who had recently accepted Islaam gave some meat as a gift to a group Muslims, who said, "O Messenger of Allaah, some people brought us meat, and we do not know whether they said Allaah's Name over it (when they slaughtered it) or not?" The Messenger of Allaah Salallaahu alayhi wasallam said to them,
"Say the Name of Allaah and eat." (Al-Bukhari no.5507)
Therefore, the general rule concerning meat slaughtered by people whose slaughtered animals are lawful is that it is lawful, unless there is proof indicating the contrary. To forbid what Allaah Almighty has made lawful is making restrictions over people without reason.
The last problem you mentioned is also from Shaytaan. In a Hadith recorded in Sahih Al-Bukhari, a man was described to the Messenger of Allaah Salallaahu alayhi wasallam - a man who would not wake up to pray on time. The Messenger of Allaah Salallaahu alayhi wasallam said,
That is a man in whose ears the Shaytaan has urinated (Al-Bukhari no.3270 and Muslim no.774)
Shaytaan sometimes makes sleep very heavy for somebody, so that he doesn't wake up for the morning prayer. You can overcome this problem by using an alarm clock or by asking somebody to wake you up.