I'm also on methadone. I have been on it twice before and gotten off it. Last time I had almost 9 years clean but relapsed and went back into the clinic about 2 yrs ago. Slowly tapering now
I say that so you know I have experience with it. Now, if you are on doses around 100 mg you definitely don't need to be using. While fentanyl is one of the few drugs that can break through heavily saturated receptors due to binding affinity, you really don't want to be messing with that on such a high methadone dose.
I'm surprised you are having cravings. But if you are, you may want to try going up a bit and trying to stabilize. I think a big part of your problem is that you haven't stabilized yet due to messing around with the pills
Cravings are just that--a 3 second thought that your brain has. It's when you feed the craving that it becomes a problem. When you have a craving you can't allow your thoughts to keep going down that road of thinking/feeling all the "good" parts. Either "change the subject" in your mind by distraction or else let your brain go ALL the way down the road past the good parts to where you actually end up--withdrawal/broke/etc. Really the best way to deal with cravings is to do something else that occupies your thoughts--read a book, call a friend, play a game, go on Internet, whatever...
Stop feeding those cravings. And stop telling yourself you only have a few days left to use. Drugs are not going anywhere. You have countless and endless opportunities to get high in the future if you choose. But for now just try to see what the methadone can do for you. I can tell you in full honesty I do not crave at all. I do wake up a little icky, but I do that on a high dose or a low dose. When my body sleeps and pH lowers that is just my result.
Give yourself a chance--not a half assed chance. And if you have to ask anything just let me know