Acting Technique

I'm not an expert on acting technique. My specialty is marketing, so for this I suggest asking an expert like an acting coach. Regardless, here are my best answers for you right off the bat:

1. I'm not sure if I'm doing my best, authentic work (especially when working with directors who aren't great).
   1. You're the only one who can decide that. I encourage you to write out a simple, 5-point checklist for yourself. What five things do you need to have done to know you did your best work? For instance, have you have asked the five W's before each scene and made clear choices on each? Have you performed from a place of passion instead of a place of "really wanting the job?" Have you "raised the stakes?" Have you stayed authentically within your brand? Did others watching you "feel" it (you can ask them afterwards if you want)? Make up your own checklist and if the answer is NO to any of your questions, then you're not doing your best work.
2. I am often not respected on set by the director or people working there.
   1. You need to be more selective with the projects you're working on. Or you need to be more demanding of respect. Perhaps you put out an air of someone who doesn't deserve respect based on how you talk about yourself or your demeanor. The simplest way to tell whether the problem is with you or the projects you're going out for is whether the people in other areas of your life also disrespect you. If they do, you know you need to check your behavior and start feeling more confident in yourself. Unfortunately feeling confident in yourself is outside the scope of this answer. You might want to join my mailing list to learn more about that and other strategies to move your acting career forward.
3. I feel like I’m unable to be creative and “play” in this industry even though I thought I would be able to. I feel like my ability to "play" is being stifled.
   1. You need to have a direct conversation with the director on whichever project this is a problem for. But even more importantly, remember that it was your fault for saying "yes" to the project in the first place without being clear on how much creative license you'd have. Next time, be sure to ask. And if you feel like the problem is in the general industry, I hate to say it but "it's you, not me." Plenty of other actors don't feel like this and enjoy what they do, so you should probably look at yourself and ask, "Am I holding myself back somehow? Am I afraid to "play" and try new things?" Chances are high that something within you is holding you back - most likely some unconscious fear. 
4. I feel frustrated and angry, and I loathe going to rehearsal when I feel like I'm stuck on sets that I don’t enjoy.
   1. Good, now remember that feeling next time you consider working for a project without doing your homework on the people involved ahead of time. Google is only a click away and you can easily reach out to actors who've worked on sets with that director or crew member(s) in the past.