Nit picking: combining meth and wellbutrin is more likely to cause hypertensive crisis (norepinephrine OD) rather than serotonin syndrome. Serotonin syndrome is pretty much the department of MDMA abuse.
The thing with meth is that if you don't have a milligram scale to weigh your doses out, you are at best guessing how much you're ingesting, and it's usually a lot more than an "effective" dose. Meth has a long half life (12 hours) too, so it will stay in your body and make you feel wierd for a while. Up to 2-3 days at worst.
Anxiety attacks like what you're experiencing are a common problem that amphetamine abusers get, it's because amphetamines release adrenaline and noradrenalin, which are the fight-or-flight / panic chemicals, and it also messes with cortisol, a major mood chemical. Usually what happens is people suddenly have an anxiety attack one day on stims, and then every time afterwards when they take a stimulant, their body "remembers" the last panic attack. The panic attacks are a defense mechanism meant to keep your body out of bad situations and away from toxins.
There are ways you can "deprogram" the panic attacks, but if you're still doing meth, that is going to be a recurring cause of problems. So you really have to stop doing it entirely. Don't lie to yourself any more, say "this time meth will work and I'll feel great", because it's not happening any more. It won't for a long time. You can count on the fact that every time you take meth, or amphetamines, or Ritalin, or cocaine, that your body will panic now. Unless you get into some serious mind-over-matter stuff you are just going to have to come to terms with this fact.
You need to throw all your meth out, get rid of your dealers #s, and have a few days where you can detox in a comfortable environment. Don't beat yourself up and don't worry about permanent damage. Humans are fucking resilient, they survive even after having railway spikes lodged through their head, or massive brain tumors, or strokes. Meth abuse will make you feel shitty for a long while but it won't cripple you if you're physically OK (no heart attacks) and you stop doing it.
Hang in there. It gets better.
