Dear Waiting Game,
"The perfect way to a perfect relationship is not to find the perfect person but to learn to look at an imperfect person perfectly."
You Are Stringing Her Along
First, why don't you ask yourself if you love this woman you are with? How do you know if you're in love? Well, quite frankly, if you have to think about it, odds are, you're not. If you don't, then you're right, after three years, you are just stringing her along, ESPECIALLY if you've told her things just to make her happy (i.e. talk of a future together, you love her, etc.).
You should sever that tie as gently as possible. Even though you may not love her, after three years she more than likely loves you. But you are correct, you'd be doing her a disservice to continue the relationship.
Take Time Off from Dating To Figure Out What You Want
If you do find yourself single after some deep thought, stay single for a while. Spend some time discovering who YOU are and what YOU want. Once you understand that, you'll be better equipped to deal with commitment, and even, dare I say it, marriage.
Quite frankly, you sound a lot like me; I had so many issues with commitment, the main one being all those OTHER fish in the sea. I mean, I'd be with one person, but what if I missed out on someone really great? My problem? I had no idea who I was or what I wanted. Once I took some time to discover that, it was a lot easier to find someone who fit me.
Explore Your Relationship
Now, on the flip side, if you do love her, ask yourself "Why?" What is it about her that makes you feel that way? If she loves you, ask her the same thing. Discover what it is you two have in common, and what it is that draws the two of you close. Explore these things, and talk about what the two of you need out of a relationship. Now this definitely requires some singular time, marriage is serious, so if you do it, do it right.
You Choose To Be Bored
Whenever you find yourself bored in your relationship, stop what you are doing and figure out why. Pinpoint it and try and deal with it on that level. You've obviously let some individual moments coalesce into a much larger dilemma. It doesn't have to be that way. The top thing to keep in mind while you're going through this is that she does not make you bored. You choose to be bored. So make a different choice. If you have gone through a lot of girlfriends, it sounds like you may be the one in need of reform.
So what it really seems to come down to is you, my friend. You need to decide who you are, what you want, and what you're willing to do in order to get it. Make the decision that is true to you, and do it with the understanding that what you do affects those around you. You should be fine.
Forget About Your Age
(P.S. PLEASE do not make your age an equation, as with most things in life, if you hurry the process, the end result will be lacking.)