I have been really struggling with the whole origins story.
I can understand that, I was exactly the same as you. If i didn't have kids asking me questions I would have never made the discoveries that I have made, which have actually reinforced my faith and made it rock solid.
Honestly it doesn't make sense. When Cain kills his brother, he is worried that others will kill him, but the Bible gives no explanation for where these other people came from?
That is a good one, and this is a common mistake that everybody makes, including myself. We make assumptions that are not supported by the biblical context, and then we get confused.
Not everything is explained in the Bible. We are allowed

to make reasonable assumptions to fill in 'gaps' however whenever these assumptions are contradicted by the bible, then we are wrong.
Naturally we can expect that the only persons Cain could have been worried about was 'family'. Who else would have cared, now we only need to consider how its possible that he did have a family
Facts (all from Genesis 3-5)
Seth was born when Adam was 130 years old.
Adam and Eve had other children.
Eve is the mother of all Living
Seth was the replacement son for Abel
What we do not know is;
- when Cain and Able were born
- when Adam and Eves other Children were born.
- when Adam and Eve were kicked out of the Garden
- when Cain got married, who is wife was, and who were the people he was afraid of.
It is reasonable to assume that Adam and Eve were partaking of marital duties as often as any couple right, and therefore strike rate of 2 kids in the first *129 years is very ordinary effort. (reduced by however many years they were in the garden)
It is more reasonable to conclude that Adam and Eve had several kids between Abel and Seth, and that several generations now exist. A simple mathematical model would show that in 100 years a very large population could exist.
Now please read again Genesis 3-5 and you will see that this satisfactorily answers all facets of the puzzle and it does not contradict any of the text.