1990 Toyota Pickup Not getting juice

I bought this truck about a month ago and it had about 340000 miles on it and a leaky radiator as far as I and my mechanic stepdad could tell. Also, it had problems idling and when I would slow to a stop the engine would die and I would have to start it back up. So when I bought it I installed a new radiator and we put some of the radiator leak-seal with the little black pellets in it to try and plug up any leaks. My stepdad also thought that there is probably something wrong with the head gasket. It ran okay for a little while and started overheating and losing water again so I put some "permanent" liquid aluminum head gasket sealant in the radiator as instructed and it ran fine for a good little while again. In the past couple of days it has started losing water fairly quickly again and overheating so I went to walmart today and got some more of the "permanent" head gasket sealant and put it in, after doing that everything went really bad. Now I can hardly get it started, it will turn over but I have to press the gas repeatedly to get it going. When I finally do get it going, the engine is making a quick vibrating/knocking sound when idling and when I try to put it in gear and let up on the clutch it starts to go but then it acts like it's not firing correctly or it's not getting gas or something. Also white smoke comes out of the exhaust when it is doing this. I can't even get it going down the road in 1st gear now and have to almost press the gas pedal all the way in at some points to keep the engine running. I understand that it could be a broken head gasket and if so I will have to take it to a mechanic, but I'm hoping that it might be something easier that I could fix on my own because I'm really struggling with money right now. Thank you for looking at my problem and for any assistance you may be able to give.