If the matter is as you describe, then you do not have to make up the prayers that you offered without being pure, according to a number of scholars, because you are excused due to your not knowing. Please see question no. 45648.
This is what seems to be the case, if it did not occur to you that ghusl might be obligatory.
Some of the scholars say that you do have to make up the prayers, because ignorance in your case is not an excuse, as you live in a land where knowledge is widespread.
In order to be on the safe side, you should make up the prayers that you offered without being pure, and try to work out how many they were. If they are so many that it is too hard for you to make them up, then we hope that there will be no sin on you if you do not do that, but you should offer as many naafil prayers as you can.
With regard to fasting, you do not have to make it up, because being pure is not a condition of fasting, and the fasting of one who is junub is valid, but he has to do ghusl in order to pray.
With regard to the waswasah (whispers from the shaytaan) that you mention, this is a serious issue, and we praise Allaah for having guided you to ignore it, because how could a person do a greater evil, which is kufr, in the hope that he will be forgiven! How does he know that he will become Muslim again after that? Something may prevent him from becoming Muslim –Allaah forbid such a fate. If this were the prescribed means of repenting then Islam would have taught it, and it would have been advised in the case of one who commits serious major sins, such as the one who killed a hundred souls.
Give praise to Allaah for the blessing of Islam and ask Him to make you righteous and steadfast; strive to learn beneficial Islamic knowledge, for it will be an honour for you .
We ask Allaah to guide us.
And Allaah knows best.