Hello and welcome!
It's great that you're open-minded and willing to explore this.
One thing I've learned is that the scariest part of poly is the anxiety drudged up by imagining how horrible it can be. Many people find that living it is much less scary than imagining it.
I had some of these same apprehensions before my husband starting seeing other people, even though I was the one who staunchly declared myself poly and our relationship non-monogamous when we met. But when he started just doing it and I saw with my own eyes that the world didn't stop turning, it got a lot better. Then when I saw how excited and happy he was from his experiences, I got happy and excited for him.
I don't think that you starting off first will make it any easier. You will still have the same feelings you would have if you weren't dating someone else, compounded with guilt, "I shouldn't feel this way because I have another boyfriend too, so it's not fair for me to feel this way, but I do feel this way, and that's bad." (it's not bad)
That's not to say that if you meet someone you'd like to date that you should wait until your boyfriend meets someone too... just go with the flow, be open-minded and flexible, be honest about your feelings but own your feelings as being your reactions and not his fault. Beyond that, talk and talk and talk