However, that doesn't seem the case as they tried to get their ex back who shares totally opposing dreams with them. You can't convince anyone to abandon their own dreams for your own, and even if you could the question would be should you.
So, for better or worse, whether it's coming from a healthy place or not, your partner is dreaming of poly in a hard way.
It sounds like they have a lot of self-worth issues to work through and a lot of growing to do before they are ready for and able to bring about happy, healthy poly relationships.
You are only five months in. You should be head over heels in love, sickening the world with your sweet, romantic gestures. If you're dealing with this many unmet needs already, it's not a good sign.
On a side note, to answer another question you asked, there are different types of poly. Some people accidentally become poly because they fall for someone near them while in a relationship. Other people do actively choose to pursue additional relationships with people they wouldn't have known otherwise. In the second case, which seems to be your partner's MO, it is commonly accepted that there is a much higher chance for success if you wait for existing relationships to be stable and steady before looking for others.