I need some advice on what could be a life changing decision.

I need some advice on what could be a life changing decision. I've suspected for some time now that my girlfriend has been having an affair. The usual signs. Phone rings, I answer, someone hangs up. She started going out with the girls a lot recently although when I ask which girls it is always Just some friends from work, you don't know them. I always look out for her ride coming home but she always walks from around the corner, I can usually hear a car driving off as she walks towards our house. If it really is a taxi why not just get dropped off in front? I once picked her cell phone up just to see what time it was and she went beserk and screamed that I should never touch her phone again, and why was I checking up on her. Anyway, I have never approached the subject with my girlfriend. I think deep down I just didn't want to know the truth but last night she went out again and I decided to check on her. I decided I was going to hide behind my car which would give me a view of the whole street so I could see which car she was getting out of. It was while I was crouched behind my car that I noticed rust on my exhaust tip. Should I take it to the dealer for replacement, or should I just take the opportunity to buy the aftermarket 3 I've been looking at. Its a big decision so I thought I would ask for your advice. Maybe I'll just try to buff it out. Help me out.