Hmm, is a period really the only possibility there? Wow, then it's good I asked for help! I had no idea. What about a semi-colon or something? *looks hopeful*
You're right I shouldn't have used a comma though.
And I see what you mean too, but you don't learn a language by being told "you're doing really well, good job" because people are afraid to hurt your feelings.
Just because English isn't my first language doesn't mean I want people to go too easy on me. Of course it's obvious I'll never be as good as a native speaker (well... I have to say I've seen native speakers with worse spelling than I but you get the point) but I'm talking about really abominable spelling/grammar and such. I don't think it's unfair to hold non-native speakers up to at least some sort of standard - as long as you use positive reinforcement as you say, they'll only learn from it and become more intelligible.