There is nothing that can't be fixed, especially since you've already done several other walls and windows before finding the problem.
First things first, are the termites active or is this old damage? If they are active, then you need to kill them before you continue, or they will just infest the repairs and you will continue to have a problem. If they are inactive, then just replace the damage and move on.
I highly recommend a product called Termite Prufe, or similar, which contains disodium octaborate tetrahydrate. This product is virtually harmless to humans and animals, something that can not be said about traditional chemical applications. If you don't care about your health or living in a chemical swill, then call in a pest company to treat for you. I will tell you that if you have an active infestation, that it will likely take multiple applications before you get it under control, small price to pay for no chemical residues AND something that is supposed to last longer than chemical applications.
As far as repair of the damage, whether or not you need a professional tradesman or not will depend on your personal abilities. If in doubt, hire a contractor to do at least the structural stuff, and you can continue doing the windows and siding on your own.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!