Software development is a field in which you formal education doesn't matter as much as in, say, science.
However, it does indicate to potential employers that you committed to a program, and were able to stick with it for a few years, and also guarantees that your knowledge rises to a given standard (less than some employers realize, but still seen as such).
The only way to combat this is to have a very strong resume and portfolio. However, from the sound of things, getting the job is not a problem for you. It's the fact that your fellow employees are not respecting your opinions. This, to me, sounds more like an interpersonal relationship problem, than a knowledge one (unless you really are inflating your own knowledge and abilities, but I can't judge that).
First of all, I don't see why you even told the others that you don't have any formal studies on the matter. Don't bring it up. If asked point blank you can evade the question, lie, or just say you don't wish to share that information - as long as the boss knows and is happy with you on the team, they don't need to know.
Second of all, in order to gain credibility you have to talk the talk, and walk the walk. If you don't know and use key programming lingo and terminology when discussing your proposed ideas, then veteran developers might come to think you really do have no idea what you're talking about.
Last by not least, stay humble. I've never had a fellow developer tell me that their opinion should count for more because they've got a higher degree than mine. However, I've also never met anyone who was right 99% of the time.
I know that I've been at points in my career where I didn't even know what I don't know (if that makes any sense). Now I'm a little more aware of just how ignorant I really am, and that there's a lot of knowledge I must still pick up before I can call myself a truly skilled developer.
Being self taught, you might be ignorant of some techniques which you don't even realize exist, let alone that you could be using them. Always keep an open mind, and don't challenge people from the viewpoint that your ideas are inherently superior. Simply debate pros and cons of any approach.