I have been with this girl for years now, we have a 2 year old daughter together. Over the years I have caught her in 4 or 5 major slip-ups which led me to believe she was cheating, but never any proof

She is slick or innocent. I'm going to have to say slick
First of all, I know it doesn't mean alot.. But my gut has always sided on her cheating on me. Every once in a while I will think she isn't because she does stuff to make me believe she isn't.
For example, she will call me multiple times per day. Certain days during her lunch break she makes it a point to call me and ask me things she has already asked me once or twice earlier in the day. Like do I need anything from the store she is going to for lunch... At first glance you think she is just makign sure I am taken care of and for some reason it made me feel she wants me to know where she is.
She makes it a point to call me and say she is there and wants to make sure again that I don't need anything after already telling her no just hours before. Typically I have noticed if I try calling back 10-15 minutes later there is no answer there or at her work phone. She always conveniently 'left the phone in her purse and didn't hear it' or some other excuse. That's weird she always hears her phone ring in her purse when she isn't banging someone.
I have done detailed checking on her cell calls... no evidence of cheating.. I have done all the obvious things including installing spyware. When she found it with an anti key logging thing she was using to clean her computer she has made sure she regularly scans her computer for spyware since. She never did before then just happened to download something cause her computer was boged down from spyware - which it really was I remember using it..
That being said I never got anything from that or her history and she doesn't appear to erase her internet history. Her e-mail accounts she knows I would have access to so I think she has another e-mail setup to use for this type of thing... She makes it a point to leave her e-mail on so I can check it any time.
She basically is flaunting the fact I can't catch her is what I get from it. She gets off on it I am prettty sure..
I wrote her a very very long e-mail going over how I think she is a great mother but I am not going to keep living with her and the sex life we have. I told her I will leave her period... if things stay like they are.
I also stated I thought she was cheating on me and I didn't think it was with a random guy... but someone she sees regularly and is giving her something I can't. I told her I didn't even care but if she was she had best quit if she wanted to save our relationship.
her response to the e-mail was "I have just now seen you start to change the last month, and I agree.. Things need to change. No I am not cheating on you and never have"
Since then she has tonight brought up when a friend was over and I needed her to check her e-mail to look for something I ordered with her e-mail addess.. Jokingly 'Don't check my e-mail what if my boyfriend e-mails me' I told her she had bigger problems than that if they were that stupid.. she just chuckled
Threw me for a loop as she typically doesn't joke about cheating at all, especially not after I brought up I thought she was a day or two before. I really think she was in her own way shoving it in my face I haven't caught her and probably never will.
Years ago I even had a PD watch her one night when I was out for the night and thought something would happen. Nothing did...
Our sex life is shit because when we sleep together she claims it is painful.. And has ever since we had our daughter. It's possible this is legit but she said herself a year ago or so it was getting better.. and mentions it less now just acts as if she has no sex drive. She does not give blowjobs and throws such a fit I don't even ask any more.. Hell she gags when she looks at cum, literally.. I will say she isn't faking that. Yeah I know, none of you would ever live with her cause of that alone.
I think this girl either has severe emotional problems or is cheating on me and gets off on not being able to be caught. Please understand I am strapped financially so I can't afford to have her followed but .. She now has a business conference she is going out of town for tomorrow morning and won't be back until late at night. She has a friend watching our daughter through the whole night and day. She offered to take her supposedly. Now it's possible she really did offer but I dunno.. it just seems weird. And yeah it is actually possible she is at a conference. But i have no way of verifying
She supposedly has another one of these conferences for a couple days up state again in a couple weeks. She is supposedly making advancements but another thing that throws me is .....
There were issues in her work place a bunch of people got fired. Now the person that originally got her the job , Craig.. is effectively her boss. Craig is the brother of one of her good friends. She hasn't talked about conferences or nada until Craig started to be her boss. Actually they came up once but then it was 'cancelled'
There have been multiple times where I felt I had effectively caught her because she would call me repeatedly letting me know where she was and what she was doing during the day .. calling excessively like she does some times. Like she wants to make sure I feel comfortable with where she is. Then I called back 5 or 6 times over 20 minutes with no answer.. No answer on the work phone either. These two times that stick out are the two times i have come the closest I feel.. to catching her cheat on me. Far from a lock though.
I think she is very good... She knows I am suspicious and knows i am net/computer savvy so she has to be careful in everything she does. I think she is, and I think she is real good at it now.
My guess is it is either a couple different people she has had affairs with for extended periods of time or she is seeing the same person now for an extended period of time Either way it isn't your average Joe one time flings.
I also don't think this person can leave their SO as I think they are married most likely with kids as well. I also think she knows them very well and is able to arrange meetings without the use of telephones, e-mail, or credit cards most likely as he has a credit card he is putting everything on.
I really want to catch her... I pretty much have no doubt in my mind she is cheating on me. I would be able to leave her even in the financial ruin I am in now.. with much more peace and I would feel better about it. She would be out on the street basically with a $1500 job and $400 a month in child support from me.. Not the life for my kid, I want her to be well taken care of.
I know it is excuse making pretty much but I WANT TO CATCH HER
Does anyone have any advice on what I should do ? I have done it all... Keep in mind this girl is being careful as she knows I am suspicous and is the only one who has a car so it's not like I can really just drive by the office and keep an eye on her.