My best friend is trying to convince me that someone I am dating is in love with me and also perfect for me in every way. I don’t get that same feeling, but my friend also knows me better than anyone else and always has my best interests at heart. Should I take the risk and listen to her or go with my instinct instead and avoid this? She won’t be angry with me, but I am second-guessing myself because I think she understands me better than I understand myself. I don’t want to miss out on someone great just because I don’t see it. What should I do?
If you can’t make up your mind, time and patience will help you figure this out. You are under no obligation to commit to a relationship and getting into one if you aren’t ready to accept it will only get in the way of it working. Why not keep seeing this person, ask for time, and try seeing this from your friend’s perspective? If your feelings don’t change after a few more dates, go with your instinct because you have to learn to trust yourself. If the person you’re dating is genuinely interested in you, he shouldn’t have a problem 
waiting either.

Are my concerns about my girlfriend’s past justified or should I assume she has changed and is ready to be a more mature individual? I ask because she used to only have casual relationships and now says she is ready to be serious.
If she says she is ready, why not trust her and go with her actions rather than her past? Sometimes, people just aren’t ready to commit and there’s nothing wrong with that. Also, what is the alternative to not accepting what she has to say? If you want to try and make this relationship work, the only way is to assume that you both want the same things. Without that, there is no future no matter how much you may want this to work.