I Am A 54 Year Old Woman. I Have To

I am a 54 year old woman. I have to urinate sometimes 4 times in an hour, but the most times when I have to urinate is when I go out like on my way to the subway or on my way home it never fails I have to stop and find a bathroom or I won't make it to where I have to go and this is all the time, sometimes I don't want to go out because I am afraid that I am going to pee on myself. I keep telling the doctors but they have not helped me with this problem yet. The last doctor said that my urine test came back normal. I think that it is something wrong with my bladder. Please help me!!!