Nancy,
Don't let one mean person derail you from your path! We all have days when we are proud of ourselves and other days we just beat ourselves up over what we could have done better. In fact, if you didn't have that insight...I'd be more worried. Trust me, there are NO perfect nurses out there...only ones who think they are!
For now, you don't need to worry about whether you will work on a med surg unit or not...just focus on continuing your education. You will have plenty of time decide where to work after you have completed your LVN program. The fact that you enjoy your CNA position is a good sign that you'd also love being a nurse. The CNA job is really hard. I've never been one but I have a great deal of respect for some I've worked with.
Whatever you do though...don't let the naysayers direct your path. We all have insecurities and occasional fears but I say...go for it!