This is a bad situation. Kicking methadone with heroin will never work. The post acute coming off 6 years of dosing is going to carry on for months. With a 60 day acute. And once that methadone is gone, the withdrawal off the dope will be horrid between uses. You will probably OD is how hard you will be going through cycles. I have tried it many times, never once worked. Ended up OD, or back at clinic. You will lose your strength fast and to kick methadone you need all the strength that will exist. Leading into it with active addiction (fending off withdrawal), there is no way as you already lost.
Don't worry about the partner. Happened to me countless times. Chick realizes what methadone really is and wants you off asap. Doesn't work that way. Also good she is gone, you don't want a girl nagging you while cleaning up. She will force you to use just so you can pretend to be normal. You clean up and will never have to bring it up again to another. (Or guy, they are probably worse lol).
You will want to get rid of the needles 100%. Different ROA in the meantime, and need to drop the tolerance. Doesn't sound good to clean up when also injecting street drugs into your body.
Personally I would drop the heroin today. Stabilize on methadone. If you can't handle dropping that dope on 10mg, then you will have no chance kicking a 6 year habit.
Or even go up to 20mg. That will hold anyone. Then go down slowly. Around 1mg a week. Then jump when around 5mg or less.
Getting off methadone is hard. You have to be really determined and face the withdrawal. Maybe cheat a few times during with a weak pill or tiny piece of sub (partial agonist so you basically stay in withdrawal), only to rest up and go on the next sprint. Eventually you start to stack weeks of clean time. Around 6-12 months you will be back to feeling pretty OK without an opiate. I am sure people have done it with full agonists but will a quick road back into hard withdrawal.
You can also go 100% clean, but definitely taper down to 1mg if possible and slow as possible. You can't reverse 6 years in just a few weeks.
The thing about methadone withdrawal is that it is very mild. You won't be puking like dope or have RLS so bad you want to slam your head into a wall. It just carries on for a long time. Hence why you need full agonists gone, especially IV dope. And now that you have been living in the full clouds of a full agonist it will quite an adjustment mentally.
EDIT: Just noticed you have been IV'ing the whole 6 years? Dude don't. You need to kick the dope 1st. The reality is you need to be on a bigger dose, stabilize so dope is erased from your memory, and then come down. If there is any vision of IV heroin in your future, stay on the methadone. When it comes time to get clean the above will help.