Normally I think a lot of people around here are too casual with telling people to end their relationships, but in your case, there doesn't seem to be much good going on at all.
You disrespect and endanger him. He treats you like crap and emotionally abuses you. Trust me, if anyone gets hanging on to a bad relationship, it's me. But you need to cut ties with your partner and get yourself centered.
You've said that you don't see your partner that much, so why not start dating around yourself? One of the reasons why some people are poly is so that they never feel dependent on the love of one particular person. You'd think that over time, trust and respect would grow, but it hasn't seemed to with the main relationship you described. So do what's right for yourself and find someone who loves you more.
If you want to keep him around because you simply can't let go yet, take some time to realize that you should only see that as a limited relationship until you both treat each other better. If it's just sex, see it as just sex.
If he really cares about you, he won't just toss you aside because you've gained weight recently. That being said, in the real world, people are attracted to certain things. Not all guys like stick skinny girls (me included. I like hips and round, soft parts as part of an overall fit, toned look), but some do. You can either decide you want to change for him - or because you want to do it for yourself...or you can love yourself the way you are and be with someone who does too. I'd at least let him know that the way he's speaking about her size hurts your feelings. It doesn't have to be an emotional guilt trip, just simply something like, "You know when you say [insert comment], that hurts my feelings, right?"
Since the relationship sounds fairly casual, this is another flag pointing towards limiting your emotional investment until you both get on the same page...or finding someone new all together. Or dating someone else while you keep limited contact with the bf.
THIS NEEDS TO BE ADDRESSED
He stuck his member in my mouth and while avoidant at first I love sucking cock so much I partook.
Am I the only one that thinks this sounds like ABUSE? That's they way it sounds to me over the internet. Forcing your cock into someone's unwilling mouth (assuming it's not just part of rough or BDSM sex) is sexual assault. Respect yourself and realize that you deserve better than someone forcing themselves on you. (Unless you agree beforehand of course
Forcing can be fun)
BEING COMFORTABLE WITH POLYAMORY
Now, to the question you asked about...becoming comfortable with polyamory. For me, the easiest way to deal with it was to think of it simply as being back in college and dating multiple people at once. It can be rougher when you're already in a committed relationship and you open it up. It can feel like you're not enough for this person who said so many sexy/romantic things to you. Remember, it's not about YOU not being enough. It's about your partner wanting variety of experience and being able to love more than one person at a time.
There are lots of links around here for external webpages that have amazing insight as to how to be a new poly person. But your issue isn't being poly.
I can agree with your bf that it does take much more extra work for a man to find a new partner than a woman. My gf keeps accounts on Tinder, OkC, Plenty of Fish, and Skout and every one of her mailboxes is filled to the brim. It's just easier because men are more actively looking...and most only want sex so their standards are much lower than women's are. (You can call this a stereotype, but I'm speaking from experience)
My question to you is...how much is it detracting from your relationship? If you're not getting what you need from the relationship, voice your concerns, but also push out of your comfort zone and give him the time he needs. Only you can know if you feel neglected. And he can either decide to spend more time with you or not. If he doesn't then it's your turn to decide if he's worth sticking around for.
[LIST=1]If I were you, I'd start dating someone else...or several someone elses. [LIST=2]If someone doesn't like you for your appearance, either decide you deserve better or use it as a catalyst for change, but don't feel badly about yourself by any means.[LIST=3] Don't let yourself be abused.[LIST=4] A partner wanting to be poly doesn't mean you aren't enough or you're inferior.[LIST=5]Yes, it takes more effort for guys to find someone new, but decide if you're being neglected and make decisions accordingly.
- Remember you've got friends here and are deserving of respect.