Relationships don't just form out of thin air. They are cultivated over time, sometimes slowly and often with many obstacles and challenges along the way. Before you can start building a healthy, long-term relationship, you must first understand that there is no such thing as one big commitment that suddenly enters you into that partnership. Instead, a relationship is the fusion of many small commitments, which form over time.
For example, think about a cake. Ignore the fact that you can just buy a cake from a bakery or get cake mix from the super market -- you're going to be making a cake from scratch. You can't just put one ingredient in a bowl and expect to have something delicious. Instead, you must add each ingredient individually, gradually incorporating them until you have a finished product. Even then, you'll have to give it some time to bake before you really enjoy the rewards.
Relationships work the same way, except instead of ingredients you'll be making small commitments or implementing small behaviors. For example, you could start by committing to date exclusively; fidelity is a key ingredient in any relationship, but it's not the defining ingredient. You'll also have to add other ingredients like emotional transparency, tolerance and patience, forgiveness, respect, and admiration. And these are only the beginning.
You don't get to have all these ingredients at the start of any relationship. Some of them might come naturally to you, depending on the nature of your partner and the current state of your relationship, but some of them can only come through time, and you'll need all of them to have a truly sustainable, healthy relationship.
Before you start thinking about marriage or any kind of big, long-term commitment, think about the small commitments you're going to have to make. Focus on those instead. They are the factors that will make or break your relationship in the long run.