You're right, you can't forbid him to do anything!

You are right, an ultimately would be a big mistake.

  You don't own him, and he has a right to make choices for himself. You also have a right to decide how you want to be treated and what you will (and will not) stand for. But unfortunately, there is nothing you can do to hold onto him.

The smartest thing you could do is to give him some time and "space." The worst thing you could do is to appear desperate or cling to him.  If he truly does care about you, he will remain loyal to you. If not, then unfortunately there is little you can do about it but move on to a healthier relationship. Remember that he will not respect you unless you respect yourself. Nothing turns a guy off faster than desperation!

So my advice to you would be to first try to relax a little!  You must loosen up your grip before you choke the relationship. The only reason you would need to question his words is if his actions don't line up with his words. For example, does he spend time alone with his friend and then try to hide it, or is he completely open about it? Does he act differently around you after her visit? Does he pull away?

Second, it sounds as though you might be revolving too much of yourself and your life around this relationship. A healthy relationship should not have fear.

I think is more her feelings of unworthiness.

The fact that it does indicates that maybe you have "lost" yourself in this relationship, so losing your boyfriend would leave your life totally empty. Trust me - I once had a habit of choking relationships by holding on too tightly. My fear became paranoia, and then my boyfriends would typically lose patience -- and respect -- for me, because I revolved my world around the relationship. I BECAME my boyfriends!  So loosen up, enjoy the relationship, and try to find a good balance between your time with him and your time with friends, job, etc.

On the flip side, you should not completely ignore your feelings

As I said to "confused", one way to know is to look for paterns.

. Perhaps you are feeling insecure because this relationship really isn't right, and deep down inside you know it. Or perhaps your instincts are right, and you sense that there is something more in this other relationship than he is indicating. So, try to think realistically about your concerns, but at the same time think about all of the good and unique qualities you have to offer in a relationship. (After all, you were created to be different from anyone else on the planet!)  Be confident when you approach him. If he is not interested in what you can offer, then move on to someone who is!