Welcome, Curious! You seem to have a good handle on this. What you describe is familar to many of us, and your assessment is a good one over-all.
could never make a decision it was always I don't know
They base all major decisions on the Watchtower doctrines. They are not encouraged to be self confident, but to avoid "independent thinking".
Was obsessed with playing poker/the dream of a poker career
That seems odd. Witnesses are discouraged from gambling, competing or seeking fame (despite Prince, the Williams sisters and other celebrity Witnesses). They are supposed to not waste time on "worldly pursuits" when they could be out in the door-to-door preaching service. This could be another source of tension that troubles him.
Would get upset easily
Personality would change , almost like he would even look like a different person.
Please read about "cult personality" vs. "authentic personality". They really can be like two different persons, one that is their own self, another that the Watchtower tells them they must be. Under stress, the conditioned reflex of the group personality takes control.
he told me his sister's friends would never come to their house
he had few friends, and most of the time they appeared to turn down his invites to his house(when he lived at home)
Witnesses are taught to not associate with "worldly" non-Witness people, but there seems to be more than just that going on. His sister's friends won't come over, his friends won't come over, and you mentioned how uncomfortable it was to visit with his parents. If even other Witnesses their age stay away, that suggests a situation where all outsiders feel uncomfortable.
He also started to lie towards the end of the relationship. I don't know if this has anything to do with JW
My opinion: Witnesses learn to lie to themselves first and foremost, while remaining unaware of doing so. That is a defense mechanism, and is how the group personality can so easily switch places with their authentic one. The same defense mechanism causes them to take refuge in the Watchtower personality when they feel threatened.
I have heard that JW can sometimes suffer from Borderline Personality Disorder. do you think that could be the case too? It seems like after he went home he wanted to end the relationship with me and he kept distancing himself, with no warning...I am doubting he went back to the church but I am wondering if his upbringing could have caused him to have problems. It just seemed so completely out of his character to suddenly leave me and I was left feeling very confused.
It's possible, but there are other possibilities too. No doubt there was much tension with his family during that month he spent working with them. You said his mother "did not seem to approve of you", and that "she wanted to help him". "Help", in her mind, means getting him back into the Kingdom Hall and away from you, the "worldly" girlfriend. Sorry to say, his ties to his mother were stronger than the ones he had to you. It's small comfort to say it's better to find that out now, instead of years later--but that's about the only comfort to find in this situation. I hope you can heal from it and find a happier life ahead.