There are two things that cause stiffness in break action guns. One is the force required to actuate the cocking levers. The other is the friction between the the metal parts of the joint. To tell which is causing you difficulties, see how much different it is, opening the gun with the hammers uncocked, such as when it's just been fired, and with the hammers set, like it's already been opened once, since being fired.
If it opens really hard when uncocked, but a whole lot easier when already cocked, then take it to a 'smith, and have him lighten up the firing pin springs.
If it's still stiff when cocked, then go to your local auto parts store, and buy some valve lapping compound. Take off the forearm, and smear some of the lapping compound on the mating surface of the hinge. Get as little as possible on the hinge pin(s) and locking lugs. Now reassemble, and open and close the action about a hundred times. Disassemble and clean very well. Presto, smooth gun. Or, take it out and shoot it a couple of thousand times.
Note: There will always be some effort required to set the cocking levers, but it shouldn't be excessive.