It’s not necessarily healthier to go meatless, but it’s perfectly fine as long as you maintain some protein in every meal. I don’t eat mushrooms, but they’re a common and good source. Walnuts, cashews, almonds and pine nuts are also good. Beans and lentils (including split peas) are an excellent source as well. Unless you’re devoutly religious, I would not go vegan - there isn’t enough study about long term substitutes for lean meats like chicken or pork and for omega 3 heavy fish. You can still maintain one meal with meat per day and be healthy.