Did I start kick-boxing too fast?

I'm 205 lbs. 5'1. Yes I know it's awful, but I got to this weight by listening to other people tell me how fat I was when I was 115 lbs, I started making myself vomit and then started using food as comfort. Anyway I am on a healthy diet-not a fad, but an actual life long diet- vegetarian -with fish occasionally. Eitherway I started kickboxing and my knees are in horrible pain and I mean I recovered from all the soreness from the first few practices, but because of my weight kickboxing is a million times harder. So I'm wondering should I lose weight like -20 lbs using the elliptical machine(low impact) and then go for a high impact like kick-boxing? My knees are giving out sometimes during the hour and half kickboxing cl, but when I do the elliptical machine I feel so much better and I lose more weight and my knees don't hurt. I'm doing it for me and my husband he deserves to have a woman that takes care of herself and I deserve to be healthy.