What do I do?
Thank you for reaching out with this question. I will try to offer some guidance based on the information you provided in the description. The first thing I want to say is how sorry I am to hear about this breakup. I think a lot of people can understand how chaotic a breakup is, especially when you felt you were on the verge of proposing! Often in times in breakups, there are more questions than answers, and closure is a concept that you can't get to by talking to the person who you loved and who is breaking your heart.
When a breakup happens when you have been in a relationship for so long it can take months or even years to fully grieve the relationship. Also, depending on the relationship itself, there may be some additional processing you may need help with. For instance, if you felt used, or if you experienced manipulation or other toxic behaviors in the relationship you may benefit from counseling to help you process those experiences and come out on the other side more healthy.
I can also see that there is possibly some issue between how you were viewing the relationship and how your partner was viewing the relationship. You were planning a future, yet your partner wanted out. Talking with a counselor about this in more detail may help you identify ways that you may have been carrying the relationship.
It is normal to feel sad and deeply hurt by the ending of this relationship. One thing that helps people is to think about the ways the relationship was not working, to begin with. For instance, you already said you felt bored and that you felt used. Those are important feelings that may indicate that you knew for a while that this person wasn't sparking joy in you and that the relationship was not going to develop.
A lot of times in relationships we fall in love quickly because of the newness of this person who also wants to spend time with us. Then after a few months, we try to make things work because we don't want to break up. When we do that we aren't really honoring our own life. Sometimes we try to make things work when the other person is pushing us away. Working with a counselor can help you identify signs of healthy and unhealthy relationships which will help you as you move forward in your life.
I hope that you have found some of these things helpful. I highly encourage you to seek individual counseling to help you get more specific help as you heal and begin to move forward in your life.