Question: iphone 6 syncing like the titanic
today I had to replace my iPhone 6 with a newer iPhone 6, so i started the backup on my macbook pro (which is about 4 years old now, almost 5) and when i finally finished updating iTunes and was about to sync the New 6; i clicked on backup device instead of start new. It proceeded to load the iPhone page in iTunes but then it stopped and had a popup that said "Unable to Sync device at this time, Session failed" to paraphrase; and now, iTunes doesn't recognize the phone at all. I don't know if my system was corrupt or anything but i've been having this issue with the Old 6 for a couple months already. what can i do to bypass this or restart the phone to start as a new phone or whatever?
iPhone 6, iOS 11.0.3, Macbook 10.10.5 itunes 126.96.36.199