I Desperately need advice

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 taxi coming home but she always walks down the drive although I can hear a car setting off. As if she has got out of the car round the corner. Why? Is it not a taxi?
I once picked her mobile 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 g/f. I think deep downI 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 veiw of the whole street so I could see which car she gets out of. It was whilst crouched behind my car that I noticed rust around my rear wheel arch.

So what should I do? Should I take it into a body repair shop or should I buy some stuff from Halfords and try to repair it myself?