This post has been a long time coming. If you read my story (found here) and struggle with doubting your salvation at all like I did (and still do at times), I wanted to share what has helped me.
First and foremost, this video.
It's about WHAT to DO when you're doubting- Look to Jesus, not yourself. I think that is the best advice, because people who struggle with this a lot are often very introspective people who think about themselves TOO much. Does that strike a chord with you? This 8 min video is excellent. (It used to be the actual video, but I guess it was deleted, so someone took the audio and put it with this picture.)
Then there's this book, which I actually have not read, but I want to. It was recommended by a trusted source, and I believe the theology is legit.
This Q&A is really good also.
If you want to delve deeper, here are some great sermons on various aspects of this.
Also, research the lives of William Cowper, Martin Luther, and John Bunyan. They have amazing stories of doubt and assurance.
I have not listed Scripture here, but these resources are all packed full of it. A great place to go in the Bible is 1 John, as its purpose is really to assure believers they are legit. It's full of lists of things to look for in your own life. Just keep in mind that assurance does not depend on perfection!
Side note: A lot of this comes straight from my theology, which is Calvinist/Reformed in doctrine, HOWEVER, I don't think you have to share my exact doctrine to get A LOT out of these resources. These are just the ones I went to in my research, because the answer to this questioning and doubting needed to come from what I already knew to be true. (And some of the reason FOR my doubting actually came from my doctrine, which is too much to go into in this post.) But in my opinion, theology is VERY important when it comes to this issue, because the opposite problem is just as common (if not more)- which is, people thinking they are saved when they are not- because of bad theology.