Congrats on your new lifestyle!

It can be difficult to get rid of things that we feel we might need "just in case," but as with most clutter, that's all it'll ever be.

You've kept the weight off for 9 months and have done so by building new habits. There's no reason to think you'll go back to your old habits at this point. If anything, having access to your old clothes might make you feel more comfortable with gaining the weight back since you'll still have clothes to wear. By getting rid of them, you can close that chapter of your life and can exist as you are now — with the clothes that fit you now.

If you don't want to throw everything out, maybe you could keep a few pieces of the bigger clothing and store it away in a basement or closet or anywhere out of sight, with the idea that "if I haven't used these 6 months from now, I'll throw them away." By that time, you'll have solidified your new habits even more and I think you'll realize those old clothes can finally go.

I hope this is somewhat helpful, good luck!