Do you think it was the tat that kept you from getting hard? She probably is pushing you away because she feels like you rejected her or were not attracted by her. All you can do if you want to continue the relationship is try to talk it out with her. As for the tat, just get some money together and take a trip to a decent studio and get it covered. I wouldn't think that would be a problem since it is a ex. Anyway, tell her how you feel for her and then leave her alone. Hopefully she will give you another chance. Have you had this problem with other girls?
I would tell her about the cheating and let her know that the tat was not the problem. It may help. Go with the "I haven't felt this close to someone since _____ and it freaked me out thinking of her cheating". Sorry to hear about the cheating by the way. Alcohol could cause it but I don't think Pot would be the issue. Not sure about speed maybe someone else could chime in there. Hell maybe go to the doc and get a temp perscription for some viagra. Probably wouldn't need it all the time but it could help you get back in the swing so to speak. If she doesn't want to give you another chance just move on. If you explain it as your problem and not a problem with her then she should relax some. Good Luck.
I second this... Except, instead of seeing a doc about the problem, if this girl gives you another chance, ask her to help you "work" on it. Get some extended foreplay going, hand job, blow job, etc. (and of course reciprocate). Slowly get back into the groove of things instead of trying to jump right in. And, working through a problem like this together can help you two feel closer and more connected and make your relationship a lot stronger in the long run.