First lesson, retirement doesn't come until you have completed 20 years of active duty.
Yes, there are some instances were personnel can retire early, some recently brought back to assist in getting end-strength down to what the Navy wants. HOWEVER, Nukes will not fit into this category.
Not calling the guy you talked to a liar, just offering up some factual information.
Second, if you got "talked into" being a Nuke, well, Nuke may not be for you. You're going to have to learn to make decisions for yourself, based on what meets your needs/wants.
Also, your enlistment is 6 years, not 8. That info is in your contract. Now, having said that, enlistees serve a total of 8 years of service, combined total including active duty, active and inactive reserve duty. The norm is for a Nuke to do 6 years, leave active duty and do 2 years of inactive reserves, usually without even knowing it.
I'm going to assume you are right out of high school and offer some advice (if you are older, listen anyway). READ EVERYTHING YOU SIGN! When you put your signature on a document, you are entering into a legally binding contract with that person or entity. It used to drive me bat $hit crazy when some new Nuke would complain about something his/her recruiter never told them. So we would go to the Personnel Office, pull their service record and lo and behold, right there in their contract that has their signature on it, was what they were complaining about. Pay attention to what you sign. Your parents should have taught you this already.
On a less grumpy note, success is within your control. You will find that most of the time, the only thing holding you back will be your own lack of initiative. Good luck and thank you for volunteering.