I think she is deceiving me

This is my personal experience I am sharing. I started dating a lady when she was about 17 years of age. That was in 2009. I trusted her more than anybody else in my life.

I cheated on her and when she got to find out, we went our separate ways for some time. I went back and apologised to her. She forgave me and we came back.

We renewed the relationship. I changed and promised not to cheat on her again. I kept my promise as time went on.

She used to work with a man at the time we got separated and the man was interested in her, but she told me there was nothing going on between them.

The man used to go to her house all the time and some times, she will lie to me, go and meet him and they will go out together.

I caught them not once, but several times, but she always told me nothing was going on between them.

This made me to find it difficult to trust her again and I have the feeling she has been sleeping with him and I plan to marry her. She always tells me she loves me.

I think she is deceiving me. What should I do?
-Abdulhak