Whether it's a boyfriend-girlfriend fling that has lasted a couple of months, or a marriage that has lasted decades, breaking up is never an enjoyable experience. There are bad breakups, and ones that are amicable, but it's all relative. One thing that can add insult to emotional injury is coming to the realization that "I am still in love but my ex is not."
How you handle the situation will depend on several factors. Regardless of what led to the breakup, or why you're feeling this way, it is something that should be addressed as soon as possible. The key is to find a resolution to your situation; otherwise you will be left with nagging thoughts about what may or may not happen.
A lot of people in your situation are afraid to talk to their ex because they are afraid of rejection. They would rather go through life fantasizing about how things could have worked out. That's really sad. It would be far better if they had found out for sure when they had the chance. Living in a fantasy world may seem to give them relief, but that relief is only an illusion. Getting back together may or may not happen, but you have to do what you can to get a definitive answer.
The more recently the breakup happened, the more important it is to get your emotions under control. You need to think clearly as you consider your options. You don't have to be a cold, logical machine, but you don't want to be a bundle of raw nerves either.
Generally speaking, you have two options: you can try to get back together with your ex, or you can let go and move on with your life.
Getting back together seems like the ideal answer when you're saying "I am still in love but my ex is not." However, before you proceed, you need to make sure that you are really in love with them. It's fairly common for people to think they are in love, when the truth is that they are just afraid to be alone. If you are sure, then you can start taking steps towards getting back together.
Letting go and moving on can be a painful experience, but it is sometimes the best option. Even if you are still in love with your ex, they may already be in a new relationship. You should not interfere in what they have now. Accept things as they really are, and do your best to move on. Finding somebody new is often the best way to get past the feelings you have for an ex.
"I am still in love but my ex is not" is a common enough complaint, but what you choose to do will depend on your situation. The advice you have been given is a good start. It would also be a good idea to seek out more information on this tricky subject.