I'd like to offer you a bitter taste of reality when supporting an addict. Many of those who do and want to be with them in a relationship after recovery and through sobriety need to understand that by the time you make it to that point the love you had for them is most likely going to be gone (specifically for IV users and meth addicts). You will have gone through soo much trauma/trust issues/anxiety which is going to carry over into sobriety whether you like it or not. That fear is always going to be there of relapse, cheating, death...... My advice is that your end goal should be to set them free to be with someone else when they fully recover. It may seem like the hardest thing to do but it's best that they start a new on a good foundation and build with someone else who doesn't carry the insecurities/anxiety/trauma that we've gone through. Sometimes the greatest act of love is letting someone go in the end. I just hope people understand why that should be done.

UPDATE: Yes I know not every relationship is going to end this way. I know their are nuances which make each situation different. In the grand scheme of things though, someone who is heavy into fentynal/meth who doesn't have any financial means is most likely going to be doing a lot of questionable acts behind your back. Some people can walk this off in the end and get past it, others, most likely the majority, it won't be so easy and that initial love you had at the start/middle/end (wherever the addiction happened) is going to be tainted.