Well, this is what is going on with you, to the extent that it can be gleaned from written material.

She does have some attraction to the guy.

He does have some attraction to her as well.

Denying those two attractions would be futile.

You have found yourself liking the guy a lot. In other words, you have come to that point in your life when you realize that people who - like you - are attracted to your partner are... well, nice folks. Not villains. Nice folks, and even to some extent similar to you, which makes perfect sense, since your being attracted to the same woman signals some commonality/similarity between you and the guy.

So register that you like him a lot and reflect on that. Maybe it is OK?.. Maybe you should not view him as a competitor?.. Maybe you can have some sort of a peaceful relationship. If you really cannot do that, then do not go into this kind of an arrangement, because you WILL feel hurt.

You wrote: "I try and not see things, like how much she smiles at him or when he says something she listens attentively".

It is futile to try and not see things. You see things because you are registering what exists in reality. Trying not to see things will not help you at all. The only thing that can help is your internal work with your mind; with your beliefs, assumptions, expectations, etc. Because see - you wrote:

"HELP ME PLEASE, I really think I found a good woman who would never hurt me and I don't like these feelings."

So you do not like these feelings. OK, challenge the root cause of the feelings. She will not take those feelings away. To the extent that you are posting here looking for some reassurance - you won't get it, because this kind of arrangement is very likely to lead to interesting developments, and it is much better to recognize that fact that to try and not see it. At the root of your feeling is your belief that unless you have monopolized her attention span, you are worthless. It is a belief in your mind, and everything else flows from that belief. Try to challenge it. Maybe it is OK. Maybe you will be able to see that she likes giving her attention to you and to the guy. Maybe it is not all rivalry.

So try it - it is all work within your own mind, independently of anybody and anything.

But if you cannot - some people cannot; some people always view romantic relationships in the context of rivalry - then do not move in; you WILL get hurt feelings every day.