Some support/help please

I keep having suicidal thoughts which have become more severe in the last few days. I have no worries in life, i'm in a job that I enjoy, i'm not in debt, i'm married to someone who I love dearly. I've got a dog too that I love so much. What I am saying which is part of what I am struggling with is that I am not depressed about my life. Infact if any of my family of friends were reading this they would be adamant that there is no way I could be feeling like this as I am so happy in life. When the thoughts of taking my own life wash over me they are so strong. I won't go into specifics but the ways I have thought of doing it are very final. I wouldn't even leave a note to explain my actions just do it. I told my wife everything on Saturday night and she was very understanding. She insisted I went to my GP today for help however yesterday afternoon I told her the feelings had gone and i'll be okay. The feelings had gone it's not like I think of doing anything all the time, it's like a compulsion that comes over me. Just this morning when I woke up I was happy, feeling better then when I was in the bath I started crying for no reason, I wasn't even thinking about anything in particular. I started thinking of a way to end it all and again without going in to specifics it would take several stages of preperation and I got to the stage of if I do this first part I know it would lead to the second step so I didn't do it. I haven't told my wife about how i've felt this morning, although she has been understanding I don't want her to worry or make me go to the doctor. I know that I need help with this but I feel my GP will just give me some 'happy pills' or not understand what I mean when I try and explain my thoughts and feelings. I have spoken to the Samaratans yesterday morning and think i'll phone them again in a minute but as good as they are at listening I realise I need actual physical help with my problem. I do have self esteem issues which when I look at myself rationally I know are silly, i'm not ugly or fat (although I could lose my beer belly lol) but these issues have made me accuse my wife of cheating or wanting to find someone else. Of course telling her how i'm feeling makes me think that maybe it will push her away and she might get sick of being someone who is mental. I have had thoughts of suicide for several years but it used to be very rare maybe once every few months so it didn't seem like a big deal. But now with it being a few times a day and it's a much more compelling feeling to do it I know I need help. I'm hoping someone reading this can understand what i'm going through and if possibly let me know how to cope. I forgot to say that I started self harming just scratching and biting myself but had strong thoughts on using a knife to cause cuts.