Assalaamu alaykum. I live in the UK, and as you may know, many shower gels/soaps/shampoos/etc here contain haram pig, beef, and so on, hence, they are possibly impure. I never used to check the ingredients of the products that I used because I was ignorant about this issue until recently.
1. Do I have to go back and check the ingredients of all the shampoos, soaps, and so on that I used in the past in order to establish whether what I put on my body was pure or impure, or can I just assume that everything I used in the past was pure and be more cautious in the future?
2. When using these soaps and shower gels all over my body, I did not take care to ensure that every area of my body was purified with water. This is because I did not know that what I was using could be impure, so I just showered without paying much attention to it. Therefore, I am now suspicious that there is impurity that remains on my body, but how can I be sure? If I rub or scratch my skin and residue or dirt comes off on my hand, do I have to assume that to be the impurity from the soap or shower gel that dried onto my skin? And if I put my hand on a sweaty or wet part of my body, is impurity transmitted on my hand? I am facing a lot of hardship relating to issues of purity; it has come to dominate my life recently. I just want to move on from this. Please give me a full answer to put my heart at ease, and kindly supplicate for me. May Allaah, The Exalted, reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

Firstly, we implore Allaah to bless you with peace of mind and facilitate your affairs. We advise you to repel and disregard such obsessive whispers; this is the most effective remedy.

You should know that the basic principle is that all things are pure (taahir) unless proven otherwise with certainty. Nothing is declared impure (najis) based on mere doubts. Hence, if you are not certain that these products (soaps, shower gels, shampoos, etc.) that you used are impure, then they are considered pure until proven otherwise with certainty. You do not need to investigate or check the products that you used in the past to verify their purity because this does not affect the religious ruling. Even if these products were proven impure and we assumed that you had performed the prayer while carrying traces of them on your body or clothes, you are not obliged to repeat these prayers given your unawareness of their impurity at the time.

Shaykh Ibn ʻUthaymeen said about the person who performed the prayer while unknowingly carrying impurity, "He is not obliged to repeat his prayer regardless of whether he knew of the impurity but forgot to wash it off, or he was unaware that his body or clothes were tainted with the impurity, or he was unaware that it is impure, or he was ignorant of the relevant ruling, or he did not know whether the impurity tainted his body or clothes before or after the prayer..." [Ash-Sharh Al-Mumtiʻ]

He also said, "Whoever performs a prayer while being unaware of the impurity that has tainted his body or clothes is not obliged to repeat it. The same ruling applies to the person who is ignorant of the relevant ruling..." [Fataawa ʻala Ad-Darb]

Moreover, the scholars held different views regarding using defiled items (mutanajjis i.e. carrying traces of impurity) outside the prayer. The Kuwaiti Encyclopedia of Islamic Jurisprudence reads:

"The Hanafis held that it is permissible to use and trade soap made of oil that is either impure (najis) or defiled with impurity (mutanajjis), and both are declared pure. This is the chosen view in the Hanafi School. The Shaafiʻis as well held that it is permissible to use the soap made of impure oil; however, they did not explicitly declare it to be pure. The following statement of An-Nawawi from his book A-Majmooʻ was cited in Asna Al-Mataalib, 'It is permissible to make soap from impure oil.' Ar-Ramli said, 'It is permissible to apply it to the body and clothes.' The Maalikis, on the other hand, made a distinction between the soap made of impure oil and that made of defiled oil. They maintained that it is permissible to use a defiled oil, but not an impure one, in other than mosque or what is eaten by human beings. Hence, they held that it is permissible to use defiled oil in lanterns to light places other than the mosque, to make soap, and for all other usages. Their statements, on the other hand, apparently indicate that it is impermissible to use soap made of impure substances such as the fat taken from a dead animal. The Hanbalis held that impure substances do not become pure by undergoing transformation or by means of fire. The soap made of impure oil is declared impure; the smoke and dust of impure substances are impure as well. This is the predominant opinion of the Hanafi school."

However, given the fact that these products are quite common and widespread nowadays, perhaps deeming their usage permissible is the chosen view so as to relieve people of hardship. There is no doubt, though, that refraining from using the products whose purity is not known to you is better out of prudence; so as to avoid that in which there is a scholarly difference of opinion.

As for your second question, you should know that these are mere obsessive whisperings. We have underlined in the beginning of the fatwa that all things, in principle, are pure. Hence, the basic principle is that your body is pure. The same ruling applies to whatever object of which you doubt its ritual purity. You should follow the basic principle and consider all things pure unless proven otherwise with certainty. Thus, you should not consider that your body or clothes were tainted with impurity based on such whisperings. You should totally repel and disregard them.

Allaah knows best.