I'm wondering if part of this comes from your own insecurities and a part of it from having views about women that are based in double standards? I don't mean to sound harsh when I say this, but it is surprising how many people, in this modern age, are still influenced by views and opinions that belong to the last century.
One thing you said struck me
-All the girls before her were rather cheap and slutty (won't go into details), she had known the vast majority of the guys she did things with for some time
Think about this. You were no different to them, the only difference is your gender. By whose comparison were they "cheap and slutty"? Is it because they agreed to have a one night stand? It takes two to have a one night stand. Is it because of their appearance? It is a sad fact of life that some women place their sole value upon themselves on their attractiveness, and this is usually related to low self esteem, the media's sexualisation of women and some men's attitudes towards them. The women with whom you had your one night stands are human beings, with thoughts, opinions, feelings, life experiences etc and are not mere objects with whom to have sex. Please don't use the word "slut" to describe them, especially when there is no equivalent word for a male. Sadly, despite great moves forward, society still has a double standard at times.
For some men, women are viewed in black and white terms, and they have a "whore/Madonna" complex. The truth about women lies somewhere in the middle. Women are as entitled to men to experience their sexuality, and their sexuality alone doesn't define them.
Please work upon your self esteem. We all have a past. We all sometimes make mistakes in our pasts. We are all entitled to, it is our perogative. We don't need to justify ourselves. We all have a present and a future. You are in a present relationship with your girlfriend, who has been honest with you and seems to have given you no reason to feel insecure.
Please work upon your reasons for being insecure, and find ways to build up your self esteem. Therapy can help.