You did better for your first time in a 7mm than I did, so don't feel like you're somehow a "failure" at diving. My first attempt at cold water diving, out off San Diego last year, ended up with two scrubbed dives. My first impression was that the wetsuit was a bit too small (felt like I couldn't breathe at all), though some here suggested it could also have been a form of anxiety due to first time in that thick of a wetsuit. Either way, the trip ended up with sitting atop the kelp forest instead of actually visiting it. The first attempt, I made it into the water, but didn't descend. Second attempt on the same boat trip, I didn't even make it off the boat.
Like you, I got to chalk it up to lessons learned. I was also a fairly new diver (I think I was up to maybe 15 or so logged dives at that point), and all my previous dives had been in the Sea of Cortez, in water warm enough that all I needed was a rashguard. I plan to try again for a visit to the kelp forest, but first, I think getting some experience in the 3mm wetsuit, and maybe some local lake experience with a 7mm, will be in order.