I appreciate the pain and struggle you are going through with this relationship, and while I don’t want to be simplistic about the differences between men and women and dating I will say there is some truth to the statement that when relationships end, men replace and women grieve.
That having been said, the insecurities and concerns you are feeling are natural in this relationship. The first thing that I would note is that the primary concern you have seems to be during the time you were broken up. During that 10-day period your then ex-boyfriend began some connections with other women. This should not be a surprise given that men will most often move on faster than women. Of course this isn’t always true, but what I am saying is that if you were broken up, his behavior isn’t a surprise.
I would encourage you to find a voice in this relationship and start creating a commitment that is based on mutual trust. Explain what you need from him in the relationship and be open to hearing his expectations. True intimacy is the result of shared vulnerability and this needs to happen with a discussion.
In your profile it indicated you were in college. I would take advantage of the counseling program at the school and see if they would be willing to have a few couples sessions. This might do wonders fo the two of you beginning to communicate your needs and expectations more directly to one another.