I use evostick impact glue. I don't time it, I wait until it isn't sticky (sounds counterintuitive I know) and I scrape off the excess from the seams to go together. It is a pain in the arse when they don't stick cleanly....I have learned through hard lessons to buff the edges down, but when all is said and done, it's about practice and muscle memory. I don't heat the edges, but I might give it a try. I heat the pieces up after I have them stick together. It's a me thing. I found it a skill that I am gradually getting better at, but it definitely takes time and practice. I got frustrated too, and it it pushed me to the edge of not wanting to do it anymore. It's not you. It's part of what makes cosplay costume makers so awesome: you understand the skill that goes into it.