Your lease may or may not prohibit this, and your being behind may or may not let the landlord escape complying with restrictions that otherwise exist. What you want is to determine what your rights are, and assert them if appropriate, quickly.
Please feel free to e-mail me, I can refer you to someone in Memphis.
say something like "So long as Tenant is not in default under this lease and is operating as a ___________________. the Landlord will not lease space in the shopping center to a ____________." If it has this language then you could lose the exclusive by being in default. It would be best to try to negotiate a lease amendment to allow you to pay what you can and stay out of default.
Some exclusives don't have the qualifier about the default - it depends on what you negotiated. You need to read the lease closely with your lawyer. And without the default language, your being behind in your rent shouldn't allow the Landlord to breach (assuming it is a breach by the Landlord).
It depends on how they described the protection that you have as to whether the UFC gym is prohibited. If the Lease says the Landlord won't lease space to a "full service health club", then you have the question of fact as to whether a UFC gym is a "full service health".
Restrictions on use are often strictly construed, so if they can argue it is a different use, you could lose.
Again, it really depends on what the exact language of your lease says. You should get a good real estate attorney and read the lease carefully with him/her.
If you can prevent the other gym from opening you may have an easier time of it.
This is not legal advice, of course, just a suggestion.
If the other store is allowed to open and you win the case, the landlord could owe them as well and it may impact your case. I'm not a lawyer but I did stay at a Holiday Inn Express last night.