Honestly, I think you didn't take the smartest road to get back on the fighting track. I don't know how long you where thinking about your "comeback", but you definitely could have prepared better. Getting back in general shape (at least a bit) before starting the MMA training takes only a couple of weeks and costs nothing. Going from 0 to 100 can damage your heart, so some endurance/cardio/BWE build up training would have been a way better start. You'd still get severe muscle ache when you go over to MMA after a couple of weeks as your muscles still wouldn't be used to the movements, but you'd lower your risk of a heart attack significantly.
So maybe you can talk with the trainer to give you three or four weeks preparation time. You can promise him to give him a report every other day so he can see that you are serious.
If you don't want to do that, but you just want to fight through the pain, see at least a doctor for a stress electrocardiogram to check your heart.
Either way, you wanted it, so "man the f&ck UP bicth" have fun in the cage and keep us up to date how it goes.