but a general piece of advice is to wear the thinnest wetsuit you can be comfortable in. If you have a very thick wetsuit, it means you need a lot more weight to submerge, then have to pump a lot of air into the BCD to keep from sinking uncontrollably, which tends to make divers swim in a "head up/feet down" position, which is tiring, stirs up the bottom, makes bouyancy control difficult, etc. If you're in cold water, of course, then I guess you need the 7mm.
Good luck, try to keep your breathing slow & relaxed, & pay attention to kicking technique...sometimes people in thick wetsuits tend to "ride high" in the water on the surface, & kick up & down lifting their fins OUT of the water, which is inefficient & tiring.