Relationship Dilemma – Competing thoughts regarding current relationship and a relationship I ‘think” I want

I have been divorced 10+ years and started dating 1.5 year ago. I have been dating someone for over a year. He treats me well, showers me with attention, and we have a very easy relationship. We both have children and they come first. We are both very understanding when one of us has activities or obligations with the kids. This was always very important to me, as I’m the only parent for my children and am very active with my kids. I never wanted to feel competing interests when it came to the kids. While it all sounds great on paper, I keep waffling around about the relationship. Is it the right relationship? Is he really what I want? I just feel so much uncertainty about it even though it should be great. It feels like there is something missing and I’m not “all in” as I feel I should be.
Fast forward . . . a man, Steve, that I have known for several years (and have really admired) is single again. I have known him through my kids activities and, once I decided to enter the dating scene, he started dating someone. I just found out this weekend that he is no longer in the relationship. My mind immediately turned to him and I can’t seem to get him off my mind. From the very first time I met him, I always thought that I’d like to date him (even before I began dating).
Cheating on my boyfriend is completely out of the question. I’m shaming myself for even having these thoughts about the other man. My boyfriend treats me with such love and kindness and hurting him by breaking off our relationship seems so unnecessary. The thing is that I I don’t even know if Steve would have any interest in pursuing a relationship with me. The only way I would find out would be to leave the comfortable relationship I’m in now. That sounds like such a risk . . . what if the grass isn’t greener? What if I give up a comfortable relationship to find out that what I had was what I really wanted?
I’m not the sort of person that jumps from relationship to relationship. I was married for 19 years, divorced for 10 and have only been in 2 relationships my whole life (my ex-husband and my current boyfriend). Having these questions in my mind is exhausting and the thoughts feel bad because I feel that, by even having them, I’m being unfaithful in my current relationship. Can you help guide me through what to do with these competing thoughts? Thanks so much for all you do!