A brief word from Ami
It's not easy to be friends, especially when you have many times been befriended by people who were only seeking to use you to up their grades, or hoping that your familial connections might help them get into a good school, and really didn't care about you as a person, only as a brain. I'm one of those who wants to see the good in people, and not the ill. I find it easier to find the ill in myself. I usually feel that I have done something wrong, not them; I also feel that surely not everyone is like that, but each new time my expectation lowers, and that was how I eventually came to believe that there was not one single person who would genuinely be my friend, that I should just avoid people all together and focus on myself only and only my studies. It was into that mentality Usagi came like a bright ray of sunshine. That is why she is so dear to me.
And from the series...
Friendship is something that does not come easily to Ami. She has been used before, and isn't sure that she can trust Usagi when she first encounters her. This is something that doesn't change in any incarnation of the show. However PGSM has been the first version of the series to really expand on the background of everyone other than just Usagi and therefore they delve into Ami's motivation or lack thereof, when it comes to friends.
All of the senshi have been burned before by people around them, Ami is the one for whom this has taken the worst toll on her self esteem. She doesn't have the fire and passion of Mars or the strength and perhaps stubborness of Makoto, she's very quiet and introverted, and when it comes to being used, rather than blaming the other party, she will blame herself, and become more insular...thinking ill of another person and she internally punishes herself.
In the anime, Ami is more inclined to worry that everyone thinks of her merely as a person who is supposed to study, although when an enemy tries to control her in R, they do so by making her think that all of the other inner senshi has turned against her. To Ami all the senshi are accusing her of being a cheat, something she logically reasons out they wouldn't do, because she trusts them implicity.
PGSM's Ami is not at that point with her friends, she is not secure enough in her friendships with the inners to have this confidence. She fears, particularly early on, that she's not being a good friend because she doesn't refer to Rei and Usagi with the -chan suffix. She wants desperately for them to like her, and goes out of her way to change herself in a way she thinks Usagi would better approve of, because of this lack of confidence she also takes it particularly hard when the senshi drift apart in the middle of the series. When she has mild feelings of jealousy she decides she must be a vile person. Her self-confidence, or rather her lack of self-confidence, is her biggest stumbling block when it comes to friendships.
It is because of her lack of confidence in the group that she is the weakest link in the senshi, and Kunzite in PGSM can wheedle that to his advantage and turn her against the others. Actually wounding Moon triggers in Darkury a return to self, but she believes because she has betrayed the senshi she is no longer worthy to be friends with them. She believes that they should all hate her because of the vile things she has done, and tries to take all of the blame for her actions, even though the others know that under normal circumstances she would never have done those things. Usagi has to give her a stern lecture on the fact that she is forgiven and the others still care for her because of who she is, not of what the Dark Kingdom made her do.
The turning evil was something new to PGSM, although the lack of self confidence is apparent even in the manga, and resurfaces a little bit in several series of the anime. In R, as I've mentioned...in S while they're studying for their exams she finds everyone ramming home to her that she should be studying and focussing on herself and not her friends and starts to lose her confidence, and almost gets beaten by a daimon, and in the first half of R almost leaves and goes to Germany until something Chibi-usa says strikes a cord in her that she should be with her friends now and that Germany can wait.