All lima beans are not created equal. First, there's a huge difference between fresh (or fresh frozen) and dried. Dried beans of any type are going to get mushy. Then, there are vast differences between the different varieties of limas. Larger varieties. like Fordhook, need longer cooking time which can make them mushy. If you use fresh or frozen green baby limas and they turn mushy, you're overcooking them.

By the way, in case your mother forgot to tell you, onions, garlic, and sweet peppers should always be sautéed before using in boiled dishes. Simply tossing them raw into boiling water is an abomination. Talk about yucky texture, I'd rather eat mushy limas any day than onion that's been boiled without sautéing first. And sautéed onions and garlic make the kitchen smell like there's serious cooking going on, even if you're only making something simple.