I'm also a young adult who has for the most part had one dog at a time most of my life. I have an 8yr old & a brand new pup. I still haven't figured out a great way to train & work with my pup without making the old girl feel ostracized. One good thing is my father lives close so I can let him take the pup outside to exercise & work with her while I stay inside with my old dog.
Sad thing is my pup now needs to sleep with us as well so I don't get those one on one cuddles anymore. I try to often reward my older dog with treats I wouldn't normal give her. I will now let her sit in my chair with me so she feels like she has more privileges than before. All the little things I can think of to reassure her she still holds my heart & she hasn't been put to the side.
Not much help I know, but I wanted to let you know your not alone in this matter. I also wanted to comment to help stay in the loop of your thread so I can check out some advice others may give you as well. I'm sure your girl knows you still love her very much, she just may not show it as well as before because of your new pup but she will come around eventually.
Sable GSD 3/2014
Stewart Australian Shepherd 5/2006
Babe Daschund 2/2006