I think that you are the one that needs space, since he has been the only boyfriend you ever had then you need to make sure that he is the one and the only way to do that is to give yourself time and space, you don't want to stay with him and then have these doubt when you are married. I would suggest giving yourself a couple of months to see if you are more miserable without him than with him but that doesn't mean date other people because you are also taking the chance of losing him forever. I think you need to communicate to him how you are feeling and tell him that you feel you have missed out because you only had one boyfriend he may understand enough to allow you that time you need but then again he may be furious that you don't feel he is enough to make you happy that is why it's important for you to make sure that you don't lose him in the process of seeing what else is out there.
Too much of togetherness too gets cramping. You don't have to be together all the time to keep your relationship intact. Be together yet give space to each other. Encourage each other to spend time for the things they like. If possible try to establish a set time for this purpose. Perhaps you could plan out every Friday night as a single night when you and your partner could do things you like to do on your own. But one thing you should always remember; Trust your partner. To keep a relationship healthy there has to be trust between the partners. There has to be an open positive exchange of trust. If this is lacking then you need to contemplate over it. It may not be that you want to see what is out there it could be a simple as getting bored of it just being the two of you. It has to be a mutual agreement. Spending time with the company of others such as other family members, friends and colleagues is equally important. You should never give up meeting your friends; family you love or give up activities you love, that should not happen. Never deprive yourself of the things you like as you will never be happy with yourself and your partner.