I had a similar experience - I have done bodypump for a few years, and thought that my form for squats etc was pretty good. When I started doing SL, I was devastated to learn that my form was no where near as good as I thought. In fact, I was so bothered about the fact I had spent 3 years doing things wrong, that I actually started to cry in the gym - not a good look!

Now, I see it all as just part of "learning". My form is now pretty reasonable, and I can lift weights that I would never have dreamed of before. I don't do bodypump anymore because I think I will get frustrated going back and am worried that I would go back to those bad habits, and there is certainly still room for improvement from where I am now, but I have got the mindset that from here on, I will learn things the proper way, and I will get better at it.

Look forwards, not back. Smile and when it all comes together and you catch sight of the perfect lunge, or well placed arms on the olympic bar, etc, you'll have a little victory all of your own!

Even after years of weight training I find my form can just progressively slip. Every few months or so I lgrab lighter weights and focus on going really slowly and properly, rather than trying to lift as much as possible. Also helps to train with a friend, someone who would be comfortable telling you if you're a little wonky.