Whathappened, your relationship with your boyfriend sounds like it's been and is very helpful to you.
You are very understandably worried about not being a priority but I wonder how often any of us are genuinely top priority in each other's lives.
My SO and I don't have other human partners to consider. But we both have busy lives. We both work. I have my canine loves who I live with. We both have lots of friends to keep in touch with (some of whom are joint friends). I have family fairly nearby. We both have interests and hobbies outside of our work that take up lots of time.
All of this adds up to a balancing of priorities. Of course, if one of us really needs the other, we will be there - but that doesn't happen terribly often.
Similarly, sometimes one or other of our friends takes priority. I have shifted things with my SO before because my friend and her husband were having problems that she wanted to talk about. I have shifted things before because one of my dogs wasn't well. Also I set aside one or two evenings a week to spend just with the dogs and make time for daily walks with them - these take priority over my SO.
Early this year my Dad died in hospital of pneumonia. At that time, being with him and my family took priority over everything else including my SO, the dogs and my work.
My SO and my close friends helped by being there in the evenings to meet for coffee and have dinner but really, family stuff was the priority.
I recently backed off from several nights out and encouraged my SO to spend time with a friend of his who was leaving the country shortly - it allowed my SO to have time and space with somebody who he feels very close to.
Several times I have talked my SO into making proper arrangements with another very close friend of his. She has a child and needs to have arrangements made in advance which is something my SO isn't terribly good at.
I suspect those sorts of ongoing calculations of priority and needs are ongoing in everybody's lives.
So - I wonder. Do you know what would help you to feel like a priority and can your partner do that? Or is it something you can only get from somebody who doesn't have another partner? Or is it unclear to you just now?
I reckon it is worthwhile thinking about. For me, I would not consider having a partner with small children. Lots of people would never consider me because of the dogs.
I hope you find some answers.