I'm in a committed relationship with someone and I know he has plans to marry me. There's another guy I've been friends with for a while and he split my last relationship up (not intentionally - I realized I had really strong feelings for him and we ended up having a little bit of a thing going on, which my ex found out about.) I still love him and always have the urge to kiss him. How do I tell my now partner. Am I poly?
I can't tell you whether you're poly; that's up to you to figure out! I have an FAQ page about that here.
Some people in monogamous relationships do have feelings/urges for other people, and consider it part of their commitment to their partner that they choose not to act on those feelings. Only you can decide for yourself whether having feelings for another person means you should try to change the terms of your existing relationship, or whether you need to take some space from that other person and accept that monogamy often requires work and sacrifice, just like polyamory.
Or, you can decide that these feelings are not something you can, or are willing to, ignore. In that case, it's worth bringing up with your partner. However, be prepared for the possibility that your partner is unwilling to be in a non-monogamous relationship with you. In that case, you'll need to decide how to act on that information from him. If it's a dealbreaker, you'll need to leave the relationship - it'll suck, but at least you learned about this incompatibility issue before you got married.
If you want to say "okay, now I know where you stand, I'm glad I checked," and continue with monogamy, be prepared for your partner to wrestle with some insecurity or jealousy, since this is being brought on by your strong feelings for someone that you've already shown a willingness to cheat with, or leave another partner for. This is a pretty fraught situation, and you'll need to really clarify for yourself whether this is about you realizing that you have the ability to love more than one person and want to be able to pursue that; or you realizing that you just really, really want to be with this one specific person.
It's entirely possible that your friend wouldn't want to be your second partner; or if you leave your current partner for him, that things wouldn't work out with him anyway. Try to maintain clear lines between "I want to try non-monogamy" and "I really want to date this particular guy" and know what you are, and are not, willing to sacrifice to pursue one or the other.