Thanks all
@Sunkyphil: Actually Kate did not want to come back, I practically wooed her back maybe because I felt I was the reason she left me or because I missed her so much... I don't know. She left me then because all the times she begged me to come to her house and see her parents, I did not go till the parents felt I was not serious and passed a vote of no confidence on me. Though she did not tell me the full behind the scenes story at the time, I remember that she kept telling me that if I did not come, it will really cause a lot of trouble but I did not believe her.

@Ceny6: When I said she helped me, there was nothing small about the help. Let me give you an e.g. For about a year that I did not have stable job, Kate was constantly splitting her salary into two and sharing with me without complaining despite that I was dating someone else.

And just to add, I have suggested to Kate about 3 times that maybe I should start treating Sarah wrong so that she can break up with me and leave me free to date Kate but Kate consistently said it is not fair that if I want to break up with Sarah, I should be truthful to her and not make her go through a heartbreak thinking it is her fault.

I know it is kind of long but I want to give you all the information might need to help a brother out:

I met Kate about 12 yrs ago when she was 16 and since I could not tell her I liked her then (she is very principled) we became very good friends. 7 years later after our respective relationships did not work out, we started dating and we dated for over 2 yrs then due to family issues, she broke up with me and went on to date someone else. Kate is very loving and caring, the kind of lady that gives and sacrifices everything, the person that understands me most in the world and she really has given up a lot to help me and because I loved her so much I was deeply hurt when she left me.
Six months after Kate left, I met Sarah who is a sweet, young lady that taught me to love again and I am her first love (she was 19 when we met and I was 29). She helped me get over Kate (or so I thought) and I loved her so much so that I already went to their parents and got introduced as her boyfriend, I have introduced her to my friends and told some of my family about her then one day about a year after Kate left, Kate called me to apologize for hurting me and that was when I learnt that her relationship had broken up.
That was close to 2 years ago, then we started talking again and to be truthful to you all I felt empty with other girls I dated after Kate even Sarah so when Kate called, I did not tell her how serious my relationship with Sarah was because I wanted her around me like a friend and I know that if she knew the relationship was serious, she would stay far away - she always believed that we were too close to remain just friends that it is either we married or stayed away from each other.
Somewhere along the line, things started getting serious between myself and Kate but I did not want to break Sarah's heart, she's so innocent and trusts me so much with all her heart, she doesn't deserve to be heartbroken so I told Kate about Sarah and how serious it was. Kate was very angry that I did not tell her on time and offered to leave me but every time she tries to leave, I always talk her back one way or the other. In this last two years, she has done a lot for me even though she knows I am in a relationship; if I should call asking for her help, she gives it readily if she can and I know she loves me very much.
Kate told me I have to choose and I have to choose soon else I will end up loosing both of them. Kate knows I am dating Sarah but Sarah doesn't know I am seeing Kate.

I am in a dilemma because I don't want to break Sarah's innocent heart and at the same time I don't want to lose Kate but she is 29 and is already being pressured to marry. To complicate it all, Sarah lost her mum a month ago so I can't even walk away when she needs me most in her life. I am just confused

Thanks as you give your candid advice.