Unstable, lying, immature. What to do.

I've always had problems with being honest. Have always been more of a doer than a thinker. Always end up doing something silly and realise only post it. And I've come to notice that when realization happens , i resort to compulsive lying and get to solving a problem by blindsiding it or running away from it. It is now affecting my future marriage, I cheated on my spouse for reasons unknown to me though I'm absolutely happy with us. I went on a holiday and cheated on him. And didn't tell him about it. He found out anyway. I did it go even after an ongoing STD I had. My fiancé is still very supportive but we're running in circles here. I resort to lying even with the smallest of things. I don't know what to do.