Hello! I have a stammer and a lisp at the same time so I know how you feel! I've had it since about the age of 5 and my mum thought it was just me messing about and didn't bother with speech therapy until I turned 16 where I begged her and told her I can't even speak to some of my friends and get embarrassed, a stammer is a real confidence killer and well speech therapy was absolutely useless, like I don't want to be mean to speech therapist but it's just didn't do anything for me.
My stammer has always been on and off, I've read books on it and tried to work out the cause, the few things I have realised is that it's different for everyone, I've just about gotten rid of mine through working out why I stammer, for me it was confidence, if I was going to give a presentation at college I'd panick, all I'd think about was 'oh crap! I'm gonna start stammering'
Admittedly now I rarely ever stammer, I might have a period where I have the odd letter I stammer on but apart from that no one can even tell I have a stammer! I'd be happy to help and give anyone tips on how I got over my stammer!