Being sure is not a criteria. In fact a lot of people have cold feet - it's nerves and anxiety over making such a huge commitment.
What you need to be sure of is your own integrity to keep the vows you make on your wedding day. Make sure that the vows are the highlight of the day, as they are most important. And make sure you do not vow anything you are not prepared to follow through on - 100%.
It is after the wedding - maybe weeks, months or years down the road when people often start questioning whether they made a mistake and start believing they married the wrong person. That's because after marriage, you get to know someone on a way different level than before you married.
When you are making this decision, ask yourself difficult questions like can you keep your vows if she becomes chronically ill, or if she gets fat, or if she decides she wants to follow a different religion, or if you find out she has different life goals than you, or when she grows old and wrinkled.... will you still be able to keep your vows? Are you willing to give up your own desires, your own needs, your own will, and always do what is in her best interest, even if it costs you everything? If you can be sure of your own integrity to keep your vows in any and all circumstances, and if you know that you are prepared to do whatever is in her best interest no matter what the cost, then you are ready for marriage, and you can "be sure".
I don't believe that something is "meant to be"...that is far too passive and I believe we have more input into how our lives unfold than most Christians want to admit.
I thought you broke up with your girlfriend?