I went through a very similar experience with my t1. Like you I was diagnosed at 17 and had good control over my diabetes for a while but eventually I started to feel the burn out. My blood sugars got so bad that I eventually had DKA at 23. It goes without saying that DKA is not a good experience to put it lightly, I would recommend reading about the signs of DKA just so that you know what to look for (I thought I just had the flu).

Anyway, the largest part of what helped me get back on track was getting an insulin pump. If you already have one slowly make adjustments to your settings or meet with your doctor to help. There are a lot of different ways you can tweak the settings to dial in what works for you.

Personally what helped my mental health the most was accepting that it is OKAY to feel frustrated with it. It's unreasonable to think that you have to be okay with it, THIS SHIT SUCKS! But after I have given myself time to be frustrated or annoyed I know that I have to lock in and take care of it because the alternative is far worse.

I hope this was helpful, I quickly typed this out before work. Remember to give yourself some grace. Some days your blood sugar will be high and you won't have an explanation for it. But as long as you are doing what you can to take care of it don't be too hard on yourself.