Most Helpful Guy
You love him, but you've been with him since you were 17. He may be having second thoughts. Assuming you're the same age, neither of you has had a true adult relationship, meaning one that started and grew while you were adults. You've only had the one relationship that started when you were high school kids. In these situations it's very common for people to have doubts about their decisions and whether they're missing out on something because they're stuck in something familiar (possibly his case) and also to hold on to something long past its expiration date because they've never faced being alone add an adult (possibly your case).
Even if this isn't what is causing issues for him, he's very clearly unhappy about some aspect of the relationship. If he can't come around or even open up enough to tell you what's wrong after six years together, you may want to seriously reassess the relationship. The state you're in now isn't a good way to start a marriage. Step back from your involvement with him for a moment and imagine two strangers in your position and ask yourself what you would advise them to do.