The reason you are having problems is a few things. At 120 most fighters will have weight and leverage on you. This can be overcome with experience, which you don't have. The mount is an extremely hard position to hold with an equally skilled opponent, if you are not raining blows down on him.
In regular randori with no strikes, I prefer the side mount or four corner position. I have submitted a lot of guys in their half guard also. The mount can be defended but it takes a lot of skill in areas that are very subtle like weight shifting and constant movement. It is like riding a bronco. Of course this becomes a lot easier if you drop a few elbows on the guy, but most don't treat our training buddies that way.
If you insist on getting into the mount, the above advice you received is good. I would also suggest that you don't sit right on the middle of the opponent, if you feel him trying to go one way, shift your weight the other way and stick out your leg for support. If he pushes up swim inside his arms and get your base again. If he pushes on your leg to shrimp to guard, scoop it off. If he is just squirming and not trying to aggressively roll you, grapevine you legs and drive your pelvis into his body while pushing one of your forearms down on his chest. Don't stay static here because you may get rolled but it is a good position to frustrate him in between things. It will help if you work on your submissions from here too. If he is fighting off your submissions he will have a hard time reversing you.
Good luck
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