Everyone else has given very good advice. To add from a personal perspective, I am the one who suggested opening up our relationship. Initially I was unsure of what I was looking for, but after a bit of searching I realised I was mainly looking for a D/s relationship, and this was something my husband wasn't really able to give me. He can see the clear differences between myself and my boyfriend, and knows (because I tell him) what it is about him I appreciate and need.
Perhaps making a list of the things you think your husband gets from you that he won't get from anyone else would help, or perhaps even asking him to do it for you? I have come to the conclusion that even if my husband could fulfil my BDSM needs, I wouldn't really want him to, because it feels right to me to keep that side of my life separate - perhaps your husband feels the same?
As far as giving you space to sleep with other people, that is by far the more fair and caring course to go down - much better than saying 'I want to sleep with other people but you can't'. You may not want to, which is absolutely fine - my husband, although he has enjoyed a few dalliances as the result of his 'freedom', is definitely not into finding someone else the way I was.
Above all, keep talking and I really hope it works out for you - in our case, our relationship has become stronger as a result of being able to be more open about our feelings.