I'm sorry to hear that things haven't gone great for you, but sometimes these things happen. I know that doesn't take the pain away, but the guys that have treated you this way, obviously weren't worth it so it's best that they are out of your life.
Having said that, it will be very difficult to trust men after how you have been treated. It's something that you will have major problems with, but it's something that you and your next partner will have to work through. It will be very difficult to do and it will take a lot of time but if the guy is worth it, he will support you the best he can. Thats the only advice I can give on that, trust builds up over time and with a good guy, it will happen.
There is no real answer to why you keep getting cheated on, plenty of people get cheated on at some point during their life. Sometimes people just can't be faithful and sadly, you keep going for the unfaithful kinds. Those who haven't been cheated on are lucky, it's a horrible thing to have happen but it's something that you will get over in time.
Not all guys are unfaithful, there are plenty of men who do manage to stay faithful to their partners and I would like to point out that women cheat too, it's not just men. It really just depends on the individual.
It will be difficult to trust men again after being hurt so much in the past but you can do it, it will just take some time but it will be very difficult to do. It might seem like you will never be able to trust them again but you will, you just need to find someone thats good for you.