Tricky. Unfortunately, if you're in a relationship, paranoia comes with the territory. You shouldn't have read her texts, because that can only fuel a paranoia you can't release, but we all do things we're not proud of when there's love involved.
I think the long and short of it is that you would have talked it out if it weren't for the text-reading.
I think she's ok to see to blokes as friends as long as she's careful and knows when it bothers you. Get too possessive and you'll push her away for good.
Play it cool, and apologise a lot for the text thing. Make it clear you only did it because you love her and don't want to lose her.
Counselling session over