My partner and I have been together for 2 years and at the beginning everything was fine. Except I kept a secret from her about a friend I had slept with 2 years previous. I told her 3 months into our relationship. She said she no longer trusted me but would try to rebuild it. We then had a break 6 months later because it just wasn't working. During this time she had been flirty texting some blokes but that was OK I could deal with that. We got back together a month later. However during a night out her cousin told me she had slept with one of her work mates on a night out. I confronted her about it and she said she didn't, her cousin did. So I asked her cousin who denied it. I decided to let it go but keep my guard up and try to deal with it however, my trust in her has gradually gotten out of control. I hack into her Facebook account and emails. The other day I was curious as to why she was being strange so I has a snoop at her messages on Facebook and found one from a bloke who she'd told to email her works email (which I can't read) this made me sick to my stomach, I really can't deal with it. We've argued constantly for 3 days none stop, neither of us knows what to do for the best.
We have had some fantastic times, and most of the time it is a great relationship (ive just told you the issues) but I need to know how I can learn to trust her. Someone help please!

You both need to stop judging each other on what you did when you weren't together! You don't have any reason not to trust each other because none of you have actually cheated, you were with someone 2 years prior to meeting her and its none of her business if who you slept with at that time, its also none of your business what she did when you were on a break.
Hacking someone's account is a felony! And you haven't found anything solid that leads to her cheating, so you both need to stop the snooping, forget the past and move on together or end it now.

Its mad that I know all of this and we've both said this but this doesn't help as to how I can restore my trust. Tips etc

There's not a sentence I can say on how to restore trust, but I know that when this happened with me and my boyfriend we made an agreement to just not talk about the past, and absolutely not snoop, it won't work straight off but you will eventually let it rest in the past and when you two start being happy again and see that each other is committed to the other things will start to be forgotten, regardless of the people you talk to on emails etc, when or if one of you have solid proof one cheated then make the decision to end it or sort it but right now you both wasting valuable time and energy on things that don't matter or might not even be true

You are as bad as she is aren't you? She freaks out on what you did before you met her, and you freak out on what she might have done, when you were broken up.

Drop it, let it go because you cannot move forward, and rebuild trust when you are looking backward and wondering what happen months ago. It has already made you a snoop, and you will never find trust as long as you have that fear of the unknown.

You have to decide how you deal with yourself, and YOUR issues, before you can deal with her. Time and actions build trust, or in your case, rebuild it. It takes time, hard work, and some courage to take a risk. If you don't have the time, or willing to do what it takes, by not snooping, and communicating honestly, and are willing to take a chance, then you may as well leave now, and save yourself the effort.

She also must be trust worthy, and only you can decide that, but if her words and actions match, then she is at least honest, and that's a good sign.