I inherited an older house that needs some work. I’ve noticed that during the summer and winter this house really struggles to stay at a good temperature, tons of draft, my energy bill every month is insane and I can barely keep the house at a normal temperature even with my ac / heater running non stop. It’s bad. How do I fix this / what’s the biggest culprit? New windows?

Start with the loft, insulate it as much as you can. If you don't have a concrete base, then floor also must be insulated well. Then windows and doors. Then get a solar system if viable, that will offset a lot of the bills.

>>2930601 (OP)
Be very careful, these old house need a lot of aeration and the walls need to be able to evaporate a ton of water that they het during winter. No paint, no cement but lime mortar, no glass or rock wool but cellulose wadding, you can use a mix of hemp and lime mortar to insulate in the interior of the house. Isolate each level from each other but allow them to have a very good aeration especially the attic

>>2930601 (OP)
Most heat is lost at holes/drafts.
Then if properly sealed most heat is lost at the roof level.since warm air rises.
Then windows.
Then walls.
Then floor.

You will need to do some investigation to understand what the house is made of. An "older house" means a lot of different things depending on region. Since you mentioned AC, I'm guessing you live in America, which means the house is probably stick frame.

Is the house one, or more levels?

Check what the insulation is like at the ceiling, walls, and floor and we will better be able to offer advice.