Sorry to hear about your situation. You are right, your agent should have present the offer in writing instead of verbally told the seller's agent what you are going to offer; unless of course, he has your agreement to do so; which apparently, he does not.
Sometimes if the offer is lower than asking, the buyer's agent might want to check in with the seller's agent and see if that number will offend the seller before presenting the offer. However, if that's the case, your agent should have consulted with you to get your approval before doing so.
However, in California, the seller's agent is legally required to present all offers to the seller; and a good sellers' agent would advise his seller to look at all offers and counter all offers instead of just refusing to see it; especially since the house has been on the market for 8 months. And the buyers should have asked the seller's agent to advise his seller not just give up on your offer but to make a counter offer back.
So, I think both agents failed in negotiation and consultation in this case, unfortunately for both the seller and you!
If you still want the same house, you might have to work with the same agent as he has the procuring cause in helping you with the house so he (actually, he brokerage firm) is entitled to the commission from the sale since he is the one who showed you the property. But, if you don't have the confidence to work with the agent again, you might go to his broker and ask for another agent with good negotiation skills to represent you. That agent can then go and talk to the seller's agent.
If you are looking at a different house, then you can go and get another agent from a different brokerage to represent you.