I feel really silly writing in about what seems like such a small problem but it's causing huge arguments between myself and my fiancé and Im afraid that I'm driving him away.

We are together two and a half years, know each other about 8 years and are due to get married this Autumn. We have everything planned for the wedding and it has been great fun. We are very happy the majority of the time and love each other very very much.
We just seem to argue over one thing; my lack of trust for him, particularly when he is out with two of his friends. These two friends have, in my opinion, a total lack of respect for women. One of them is in a long term relationship, and has cheated on his partner on numerous occasions. The other seems to think he is still an 18 year old and goes out with the sole purpose of finding a one night stand - he has never been in a relationship, and he is crude towards women, constantly making crude comments about women when in my company and in front of my fiancé, asking him to comment on women while I'm there. This upsets me, and I have said it to my fiancé, who has asked him to stop doing this.
They are constantly inviting my fiancé on lads nights out. Whenever he does go with them he stays out until all hours of the morning, with no contact to let me know where he is. I must say these nights out do not happen very often but when they do I get very anxious, wondering where he is and what he's up to.
I do believe that he has never cheated on me, which makes me feel silly for being so paranoid. He also checks out other women in front of me, which upsets me sometimes.

We were going out for 8 months about 6 years ago. He was going out with a girl for a year after we broke up, and he cheated on her once with me towards the end of the relationship. He said that he did this because he was still in love with me and he knew that she had cheated on him.
Im really confused. Why can't I forget the past and why can't I relax and just go with the flow of the relationship? I have no reason to think that he will cheat on me, he gets upset when I try to talk to him about my insecurities and I'm afraid that I'm driving him away. He says I need to forget about his two friends and he is at a different stage in his life and he has no interest the scene that they are into. He has been friends with these two people all his life so I feel bad also that I don't like them as I know he would prefer it if I liked them. I just truly do not trust them, nor do I trust him when he is out with them. They are crude and unfaithful even when I am out with them, so I can imagine what they are like when I'm not there.
I have never had these kind of trust issues in past relationships. I do think my trust issues are irrational, given that he has never cheated on me. However, I cannot get these thoughts out of my head and it's becoming a real issue in our relationship.
I really love him, and we have no other issues in any part of our relationship, he treats me so well. Most of the time I think its all my fault and I just need to get over it, but it keeps playing in my head.
What can I do to help me get over this lack of trust? I really want to try and get over this and move on.

It isn't a 'small' issue. Trust and communication are two things that help build a healthy relationship.

I think you need to relax a bit. You are allowing yourself to blame him for how his friends act. He can't control them any more than they can control him. It sounds like your boyfriend has matured and has found what makes him happy. Just because he lets off some steam every now and then with them, does not make him a different man than the one you have come to know and love.

Talk with him about your concerns, but don't try to make him choose between you and his friends. Be honest with him about thinking they are jerks, but don't paint him with the same brush. Learn how to joke about how stereotypical they are acting. Don't let them put a wedge between the two of you. It makes a great plot for 'coming of age movies' and romantic comedies, but it doesn't work too well in real life if you are aware of what is going on.

Let him have his nights out with the lads ('boys' night out seems appropriate) without needing him to check in with you. Trust him. Plan activities with your friends for the same time. Keep yourself busy with things you like to do instead of worrying or jumping to conclusions.

I think the roots of your fears is because he showed you that he will cheat (though it was with you, which doesn't matter), and his unsavory friends.

I doubt you would feel that way if you liked them, or they had stable lives with girlfriends.

That's a lot to overcome, but if you make a conscious effort, when you have those fears, to think long and hard before you ACT or SPEAK you will stop a lot of fights, and get yourself under better control, and through repeating this exercise, you can better deal with those fears, that only you understand.

Be patient, as this is a process of changing your thinking to deal with your fear. It does take time and a bit of work, as you learn not to get carried away by your own FEARS.