My girlfriend is cheating on me. Help!

I've been with my girlfriend for 3 years, we are both 23. Our relationship has been healthy and happy, until now.
A few weeks ago I noticed my girlfriend was spending more time of her phone than usual. (Sending and receiving texts)
I did mention it to her, and she said she was in a group conversation with her friends from uni and that they are all planning on meeting up sometime soon, hence why she's been on her phone so much. I completely understood, and that was that. Until, one day whilst we were sitting at the table doing work she needed the toilet and left her phone unlocked on the table with me. I saw the opportunity to go on her phone and make a funny Twitter and facebook status on her account, as you do.
As I was doing this I saw she received a message from a guy called Aaron. Now, at this point I was a bit confused because we are both fully aware of each others friends and colleagues and never have I seen or heard of someone called Aaron. Naturally I opened the message to see who this guy was (Its worth noting that I had no intention to 'catch her out', and I never thought for a second she could be cheatin on me, I was just curious.)
The message said "Hey hot stuff, still up for next Saturday?". Obviously I wasn't expecting that, and alarm bells started to ring as she told me she was going to see her friend, Amy, from university, for the night on Saturday. I scrolled through the messages from Aaron, and things just got worse. They were talking about sex positions and what they want to do to each other and had even sent nudes. My heart sank! It wasn't until I reached the top of the conversation I died inside. It turns out that two weekends ago, they had slept with each other, she had told me she was with another uni friend. Instead she was having sex with this Aaron in a hotel paid for with my money.
I heard the toilet flush and quickly put the phone back to how she had left it. I said nothing to her about it, and still haven't. It's been two days since this happened.
I don't know what to do. Part of me wants to follow her next Saturday, and catch her in the Act. Another part of me wants to tell her right here right now. Obviously this is the end of the relationship. I just want to end it in a way that puts me on top and in a good position. Help!

No wonder she's cheating on you. I'd want to cheat too if my boyfriend has the sense of humour of a 12 year old.

Thanks for the advice guys.

She shouldn't be upset over the fact I've been on her phone, as shes done it to me before.

I just want to make her feel an inch of what I feel.

I've been with my girlfriend for 3 years, we are both 23. Our relationship has been healthy and happy, until now.
A few weeks ago I noticed my girlfriend was spending more time of her phone than usual. (Sending and receiving texts)
I did mention it to her, and she said she was in a group conversation with her friends from uni and that they are all planning on meeting up sometime soon, hence why she's been on her phone so much. I completely understood, and that was that. Until, one day whilst we were sitting at the table doing work she needed the toilet and left her phone unlocked on the table with me. I saw the opportunity to go on her phone and make a funny Twitter and facebook status on her account, as you do.
As I was doing this I saw she received a message from a guy called Aaron. Now, at this point I was a bit confused because we are both fully aware of each others friends and colleagues and never have I seen or heard of someone called Aaron. Naturally I opened the message to see who this guy was (Its worth noting that I had no intention to 'catch her out', and I never thought for a second she could be cheatin on me, I was just curious.)
The message said "Hey hot stuff, still up for next Saturday?". Obviously I wasn't expecting that, and alarm bells started to ring as she told me she was going to see her friend, Amy, from university, for the night on Saturday. I scrolled through the messages from Aaron, and things just got worse. They were talking about sex positions and what they want to do to each other and had even sent nudes. My heart sank! It wasn't until I reached the top of the conversation I died inside. It turns out that two weekends ago, they had slept with each other, she had told me she was with another uni friend. Instead she was having sex with this Aaron in a hotel paid for with my money.
I heard the toilet flush and quickly put the phone back to how she had left it. I said nothing to her about it, and still haven't. It's been two days since this happened.
I don't know what to do. Part of me wants to follow her next Saturday, and catch her in the Act. Another part of me wants to tell her right here right now. Obviously this is the end of the relationship. I just want to end it in a way that puts me on top and in a good position. Help!

If I were you, I'd act oblivious until just before she goes out saturday, and then just before she goes out say something like "tell Aaron I said hey", this will shock her and confront her last minute, as well as giving her the impression that you don't care, which will probably hurt her too

"tell Aaron I said hey"

No wonder she's cheating on you. I'd want to cheat too if my boyfriend has the sense of humour of a 12 year old.

If I were you, I'd act oblivious until just before she goes out saturday, and then just before she goes out say something like "tell Aaron I said hey", this will shock her and confront her last minute, as well as giving her the impression that you don't care, which will probably hurt her too