Just remember, not all wetsuits are created equal. If you were cold in a 7mm, that might mean you need to step up to a semi-dry or dry suit. But, it might just mean you need to try a 7mm that fits you better, has better seals, and/or has better seam construction.
A good-fitting and good quality 7mm with a separate hood MIGHT even be warmer than a semi-dry, if the 7mm fits well and the semi-dry doesn't.
Don't get me wrong. I'm not pushing in either direction. I'm just trying to make sure the OP has all the relevant info to consider while making a decision.
For that matter, I can look back to when I did my OW certification. It was in November, in a cold local quarry. I was in a rental 7mm and whatever hood I could find in the bag they put out for us that sort of fit. And 3mm booties. I don't think we went deeper than 20 or maybe 25 feet. I was cold! But, now I can wear a 7mm and a hood in the same water temperature and be very comfortable. The differences are that now I have a good quality wetsuit that fits me well, a good hood that fits me well, warmer booties, and I have 3mm gloves that I would wear. A hood (obviously) and gloves make a big difference - to me, anyway.
Also, I have learned that I often feel a little cold when I first get in, but then my suit gradually floods, the water inside warms up, and then I'm comfortable. If I got out after a relatively short time, I would probably think I needed more exposure protection for whatever water temp I was in. And in reality, just giving it a little time is all it takes. Just something else for the OP to think about when evaluating the experience they have now and deciding what to do next.
