Difficult to finish work started at a previous lab
Working on projects when you are away is difficult in most situations, especially yours. Two possible options to finish them include: 


Adjusting to your PhD workload until you are no longer overwhelmed. When you're at that stage, you can try spending several hours a week on those old projects of yours. This may require saying "no" to some additional projects or opportunities at your new place - at least for the time being.
If possible, taking some time off to go back to your former university. Physical presence will greatly accelerate things, and you won't have to rely on the email so much. Plus, you won't be distracted by all the other things you need to do.


Finally, you can try talking to your old advisor and admitting these difficulties. The two of you may then decide to let it go or figure out a solution together. 

Remember, though: right now, it is essential that your PhD work goes well, since it is key to your future career. Everything else can wait. It's annoying when things don't get finished, but this might be a sacrifice you have to make - at least for the time being.