Not sure if I can go on

Hi guys,
I know I have been a regular poster here but I have never felt this way. I accidentally cut a wire whilst mowing the lawn earlier and part of me wishes it had electrocuted me and finished things off. I am an absolute letdown, especially to my girlfriend or at least it feels that way. I have so many intrusive thoughts coming into my head, I cannot work out what is real and what isn't. It is very hard for me to formulate my thoughts and get them onto paper but it simply follows on from other threads I have posted here. In short I am beginning to think I am some sort of sex addict or pest. I have always been a regular masturbator since my mid teens and this may partly reflect on my behaviour. I have had thoughts re other women whilst I have been in my relationship with my gf, I guess I was having these thoughts when single and I paid no attention to them then, since being single there were 'ok'. Now I am in a relationship it seems the more I really don't want to look at women and that includes from a schoolgirl age right up to say early fifties, the more the urge to look comes upon me and it is killing me. If I do look at a girl and then smile at them I chastise myself so much. These urges are even more damaging since I have found myself fantasising about her housemates, in particular one of them who is only 19 I think. I have thoughts along the lines off, perhaps you should split with your current gf and get with her, it really is merely a sexual thing, I dont think she is very intellectual shall we say, like my girlfriend is. In addition to this, I have found myself getting along really well with a girl who works in the same office as me, we have the same sense of humour, and part of my thinks the attraction is down to this humour and my therapist saying people are attracted to others who are a reflection of yourself. It causes lots of thoughts to come into my head and the other night whilst lying in bed, the thought perhaps you should message her and cheat on your gf!!!! What kind of person thinks that. I really hope these are intrusive thoughts and it is like I have a split personality, part of me the normal sensible thinking, and the other, a person who likes to torment the other and think everything that is wrong should be done. I must stress, not at any point have I cheated on my gf, but in my head I am killing myself. Part of me is thinking of breaking it off, because she deserves better or a boyfriend with a more stable head. She is really into me, more than I into her or at least currently like that. It would break her heart to end things and I think I would be broken as well. Part of me thinks I need to stay single for a while until I calm down.