Don't know where you live, but in the USA just about any hardware store will sell EXTERIOR grade door handles for somewhere between 25~50 bucks on the lower budget end. Assuming you've got cash or a friend who can loan/give you some money. (Or if there are abandoned buildings nearby... you know... grab a phillips screwdriver and a flat head screwdriver and make magic happen).
Installation is very easy since you're just changing out door handles. There will be a hex key for changing the handedness of the handles and all the screws should be included along with (typically) easy to understand instructions. Should be two long screws that will hold the inside/outside handles together and (possibly, sometimes things work out and you don't have to change out the strike plates) two short screws that hold the strike plate to the door and two more to hold the opposite strike plate on the door frame. Just remember to put the keyhole facing outside of your room and the handles oriented so that they are pointing in the correct direction actually shut the door and you're pretty solid.
(If the door opens in to your room, you can might be able to ignore this next bit.)
If the gap between the door and the door frame is big enough it is possible to use something like a butter knife to work the door catch back into the door if you're able to pull on the door. Make the gap smaller if you can, layers of elmers/wood glue by itself or layering paper and glue to a proper thickness before gluing it to the door frame.
Also, if you're still worried about the door being jimmied open this way, always remember, "Lube, lube, lube." Lube up the door catch with anything you can get that doesn't turn sticky (you're probably going to want to wipe off and reapply old stuff on a somewhat regular basis. Petroleum jelly, cheap chapstick, mineral oil, a dab of motor oil or 5-in-1 oil.
If you've got some beefier wood working tools accessible, you can put in a dead bolt (but you're going to have to new holes into interior doors.)
If you've got gorrilla/super glue handy you can probably glue the hole shut that works as the "emergency release" for the interior grade lockable door handles.
If you're just worried about privacy for yourself while you're in the room, you can buy and install those slide chain things for way cheaper than a new door handle. Downside is, most interior doors are hollow core so there isn't much for the mounting screws on the door side to sink in to and some hard shoving or kicking can work them loose.
Also, which side of the room are the door's hinges? Are they inside the room (and protected by the locks/shim/etc when the door is closed) or outside the room? If outside the room and visible when the door is closed, see if you can tap the pins out from below. Older model hinges can have a pin that slides through both sides of the hinge and be knocked out with a screwdriver and hammer (possibly defeating any attempt to jam the door shut or replace the current door handle with a more secure type). Newer model hinges are machined together as a single unit and its either very hard or impossible to hammer the pin out without severely damaging the entire hinge. If you find you can hammer out the pin from the bottom (and there's no way to pull the pin up from the top) you can try to superglue a small piece of metal on the bottom of the hinge. It won't stop anybody who really wants to hammer out the pins but it will make it more work.
Yeah, that's some dodgy behavior (unless you do dodgy things and keep getting caught). You might want to casually look into finding some safer spaces to inhabit.
Don't know where you live, but in the USA just about any hardware store will sell EXTERIOR grade door handles for somewhere between 25~50 bucks on the lower budget end. Assuming you've got cash or a friend who can loan/give you some money. (Or if there are abandoned buildings nearby... you know... grab a phillips screwdriver and a flat head screwdriver and make magic happen).
Installation is very easy since you're just changing out door handles. There will be a hex key for changing the handedness of the handles and all the screws should be included along with (typically) easy to understand instructions. Should be two long screws that will hold the inside/outside handles together and (possibly, sometimes things work out and you don't have to change out the strike plates) two short screws that hold the strike plate to the door and two more to hold the opposite strike plate on the door frame. Just remember to put the keyhole facing outside of your room and the handles oriented so that they are pointing in the correct direction actually shut the door and you're pretty solid.
(If the door opens in to your room, you can might be able to ignore this next bit.)
If the gap between the door and the door frame is big enough it is possible to use something like a butter knife to work the door catch back into the door if you're able to pull on the door. Make the gap smaller if you can, layers of elmers/wood glue by itself or layering paper and glue to a proper thickness before gluing it to the door frame.
Also, if you're still worried about the door being jimmied open this way, always remember, "Lube, lube, lube." Lube up the door catch with anything you can get that doesn't turn sticky (you're probably going to want to wipe off and reapply old stuff on a somewhat regular basis. Petroleum jelly, cheap chapstick, mineral oil, a dab of motor oil or 5-in-1 oil.
If you've got some beefier wood working tools accessible, you can put in a dead bolt (but you're going to have to new holes into interior doors.)
If you've got gorrilla/super glue handy you can probably glue the hole shut that works as the "emergency release" for the interior grade lockable door handles.
If you're just worried about privacy for yourself while you're in the room, you can buy and install those slide chain things for way cheaper than a new door handle. Downside is, most interior doors are hollow core so there isn't much for the mounting screws on the door side to sink in to and some hard shoving or kicking can work them loose.
Also, which side of the room are the door's hinges? Are they inside the room (and protected by the locks/shim/etc when the door is closed) or outside the room? If outside the room and visible when the door is closed, see if you can tap the pins out from below. Older model hinges can have a pin that slides through both sides of the hinge and be knocked out with a screwdriver and hammer (possibly defeating any attempt to jam the door shut or replace the current door handle with a more secure type). Newer model hinges are machined together as a single unit and its either very hard or impossible to hammer the pin out without severely damaging the entire hinge. If you find you can hammer out the pin from the bottom (and there's no way to pull the pin up from the top) you can try to superglue a small piece of metal on the bottom of the hinge. It won't stop anybody who really wants to hammer out the pins but it will make it more work.
Yeah, that's some dodgy behavior (unless you do dodgy things and keep getting caught). You might want to casually look into finding some safer spaces to inhabit.