It has been said before, but you can't have a very successful relationship until you are comfortable with yourself first alone. Many times we define ourselves through our relationships. This is a bad way to be as if the relationship should fail then it could only be because of you
The key to a good relationship is to know who you are and your relationship is an extension of that (and not the foundation). This does not mean that the removal of the relationship through outside means (i.e. death) wouldn't rock you, but the core of you will still be there. Some people are never ready for this. It took me about 6 years into our marriage before I figured this out. A close friend of ours died this weekend (he was 77 and had a heart attack). He was married to a woman who was 52, and she knew this was going to happen eventually. She regretted not saying, "I love you" the last time before he died (it was a sudden death). My wife said to me that isn't it good to know that even if we died that the other would have no regrets about having said I love you, and I had to think that she was completely correct. We say it every day with meaning, and I would have no regrets if something took either one of us.