Maybe you have build up from the heat protectant you used? Silicones make my hair straighter and less defined, I had a few horrible lank weeks recently from an accidental silicone. That and mineral oil could be locking moisture out of your hair, you can clarify with cocoamidopropyl betaine you don't have to sulphate.
It's always a good idea to have a trim after damage is caused. Could also try a coconut oil deep treatment? Are you really using hairspray on top of little to no conditioner? Hairspray can be quite damaging.
P&G Beauty & Grooming | Hair Spray Accumulation
It could be any or all of the above. My hair HATES mineral and castor oils. Causes big issues for me--hair looks dry and greasy at the same time. Usually, when I use an ingredient my hair doesn't like, coco b just doesn't seem to remove it. I have to use something stronger.
I used to do a sulfate shampoo once a year--just because I thought it wasn't a bad idea. I stopped doing that, but had to do it about two months ago. I did a gelatine PT two weeks in a row and had used the SM Smoothie a few times (in the previous months.) My hair looked bad. Don't know if it was just TOO much. I have gelatine in my okra gel--1 envelope to 4 cups gel, rather then 1 envelope to 1/4-1/3 cup water. While I use really rich products, the SM Orange & Hibiscus products were just too heavy for me. I thought cleansing with soap bars removed it--but maybe not.
I get my hair cut every six weeks, so can't relate to that. I do think that a cleanse with a stronger surfactant is in order, a trim--and stay away from mineral oil. The cleanse can't hurt. A cut will shape, redo your layers and get rid of some damage (if you have it.)
Then, you can start working with products to help with the weather. Weather in most places is getting better. Dewpoints are becoming "ideal." If anything, you hair should be "growing," not drooping.