New Vet Assistant and I'm having trouble with restraint

Hello fellow veterinary workers! So I'm new at being a vet assistant. I'm taking a course and haven't graduated, but have been doing some clinic hours at a facility. I'm having some trouble with restraining animals. I've only worked with dogs and cats (mostly dogs). I don't really understand how I'm supposed to restrain the animal so they don't bite, but also not restraining so much that they get more stressed. I'm doing the restraints like I've been taught. I feel uncomfortable doing a lot of them (not like in a moral way, like I'm physically uncomfortable and feel awkward)

The animal already is stressed. As soon as you start a procedure, they start thrashing. I have to hold them tighter to try to keep them still, but holding them tighter makes them thrash. Idk if I'm just a pushover and they feel they can get away or what. I'm also short and not super strong, does that play a part as well?

Any videos I watch show the techniques being performed on calm animals. I don't really understand how to perform restraints on uncooperative ones.