What bit do you use? Is it adjusted correctly? You should be seeing only the beginning of a single wrinkle at the corner of Murray's mouth. Any tighter and the headstall pressure could be causing him to throw his head or otherwise "misbehave," any looser and the bit can be "washy" in his mouth, something most horses don't like much better than a too tight bit.
You've already gotten some good advice on training him to stand still for mounting, but at the end of the day I think your mounting issues are a symptom of the lack of groundwork and control on the ground. Can you be more specific about what he does when he "gets mad and tries to fight you off?" What are you asking for when he exhibits that behavior? IMO, get control of Murray's shoulders, hips, ribs, etc. on the ground, and that control will carry over to mounting time.
I agree with Barry Godden that you need to spend more time with Murray, if at all possible. Contrast that with the 16 hours a day that he spends eating (if he's turned out or has free choice forage), the 4 hours a day he sleeps, and the remainder of the 24 hour period that he spends exercising (if outside), or staring at the wall (if stalled). I also have a suspicion that if you see him even every other day, and groom him, lead him around, hand graze him, otherwise bond with him, you may find that groundwork becomes less of an issue.
Hope that was somewhat helpful, and good luck!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown