I don't think it has to feel forced. The show has at least two seasons ahead of it, and the crew has known since at least the end of last year. The writers have been excellent at planning things out so far, I trust them to not start fucking everything up now.
The obvious course of action for the story to take at the beginning of Season 3 is for Marco to chase after Star. He spent 16 years on a quest to replace her pair of scissors, there's no way he's not going to do everything in his power to help her, even if it's just to get closure between them.
Jackie is Star's friend, and Jackie is a nice person, even though Star's confession definitely shocked and hurt her. She'll be understanding that Marco wants to help his friend, the question is how understanding and where the writers want to go with the bigger picture of the story.
The main question is whether or not Marco is going to Mewni alone or if Star's other friends will help. If it's a party deal, I would expect Jackie to come along, but Marco putting himself (and the group) in danger on Mewni would create more strain in their relationship. Again, depending on how they want to take the story, either she would eventually get fed up and ask Marco to just come home with her, and he would refuse, or she'd stick with him and their relationship would be a continuing issue with Star through Season 3. I think the former is more likely.
If Marco is going after Star alone, then it's a lot more likely he and Jackie break up before he leaves. Jackie is Star's friend, and she's been nothing but chill and understanding up to this point. Star's confession clearly hurt her, but it would be pretty out of character for her to bitch Marco out about it. They may not even explicitly break up, she probably trusts him enough to believe him if he says he doesn't feel that way about Star.
The most likely way I see them ending is with a hug goodbye and a killer line from Jackie like "I wish I had a friend like you, Marco."