I understand how hard it can be to break those habits. I've had this issue with addicted videos games. Like you I tried to stop many of them cold turkey, but I would eventually start playing that addicting online game (world of warcraft, League of Legends, etc. ) again and fall into the same trap
I'll tell you what I did and hopefully it helps you, but I had to quit the games several times cold turkey. The more I did it, the longer and longer I stayed away from it. Eventually I stopped playing those games. Yes, I found other games I became addicted to, but what always works for me is just cutting the thing completely out of my life so that I don't give myself the opportunity to become re-addicted to it again.
If instagram is really a problem for you, then you will have to keep cutting it out. You went half a year without using it, and that's great! don't look at that as a failure, look at it as a huge accomplishment!
I don't know if you create a whole new account when you re-join the app, but if you keep using the same account, then make a very long and random passwords that you'll never be able to remember and then "lose" that password so that you'll never be able to get onto that account again. Also, make the password recovery hard as well.
Then when you lose access to those social media accounts, do you best not to think about them. And if you do have thought about them, just do your best to change the subject within your mind and distract yourself with something else. The longer and longer you go without accessing those sites, the easier and easier it will be to not access them at all. But you have to make the choice and do your best to commit to it. And if you do slip and re-join the sites, don;t beat yourself up over it. Forgive yourself and just try again until you succeed. If you can go 6 months, then you can go a year. And if you go a year, then you can go two, and so on.
Just note that whatever you focus on the most is going to gain influence in your life to the place where you'll begin to be over taken by it. For example, if you are struggling with lustful thoughts, you need to stop those thoughts of lust before they even take place in your mind (once they enter your mind, reject them and think of something else).You cannot allow those though to run wild, and you cannot afford to entertain them either and allow for those desires to grow more and more within you. Because the more you think on and entertain those thoughts of lust within your mind, the harder and harder it become to resist the temptation of going onto the dating sites and instagram and acting on that lust.
You'll also have to guard your heart and separate yourself from things that can trigger those thoughts of lust. So guard your mind my friend, and ask God to give you supernatural strength to overcome this stronghold.
I hope this helps, but I'll also be praying for you, and come against the strong holds on your mind, and also come against the spirit of addicition that is over your life.
God Bless.