First, follow the links that Gala gave you, as well as listen to her advice.
From my perspective, I have to ask myself: Am I unsure about our actual relationship or do I just not have enough going on in my own life?
To vastly oversimplify...
If it's that you're unsure about your relationship, figure out what you're afraid of losing and communicate with your partner about it. Are you worried about not getting enough time with them? Are you worried they won't be there in a critical moment? Are you worried that you're not fulfilling their needs? Really dig deep and express what you're afraid of with them. Hopefully they'll help you resolve those fears.
The flip side of this, and the one that I really had to own up to, is that sometimes the problem isn't that you're afraid of losing them, it's just that you don't have enough going on in your own life to fill the void while they're gone. If you're sitting a home, pining away while they're on a date, and have nothing to fill your own time with, it's very easy to have dreadful thoughts about problems with your relationship or fears about polyamory. But if you do have your own interests, goals, activities to round out your life, that's a much better situation to approach the new issues that come up with polyamory.
Some will say you should find your own new person. That works for some, but don't feel like you have to do that. There are lots of great poly relationships where one partner stays monogamous while the other is poly...or even both partners are poly, but one just doesn't have a partner at the moment. Everything doesn't always have to be equal and forcing it to be equal can be unhealthy and artificial.
I sincerely hope you feel better. Be healthy and the right result with come.