Seems like the rootstock has takenover the grafted scion in your tree and hence you are seeing the extremely thorny branches and insipid fruit of the rootstock taking over the tree.
I suggest that you take a really close look at the bottom part of the trunk of your tree to figure out whether you have a graft line visible. Also check if there are two "types" of foliage and two types of fruit. If so, then the thornier part with the useless fruit (which I am assuming has taken over the tree) needs to be pruned to below the graft line. In time the scion will take over again and your tree might get healthy. On the other hand, you can get rid of the existing tree and make a fresh start to save time and effort. Keep us posted.

Yes, agree completely with ashleysf.
Looks to me like Rough Lemon (Citrus jambhiri)- often used as rootstock - has taken over. A google will give plenty of info on this variety.