TF2 is not an easy game, TBH if you are not accustom to FPS games it is something that you will have to grind for a good thousand hours or so before you really can even start to stack up against some of the comp players. It doesn't help that you do not get much consistent playtime in the game either as that is a very important aspect. It also sounds like you need to upgrade your rig, having freezing issues and sub 60 fps is not doing you any favors.
If you want to get better at TF2 it sounds like you need to go back to to the basics, learn how to properly aim, learn how to use the games movement, try to understand how the game works at its core. That may be hard if you do not have time but it is probably your best bet at this point.
It is also possible that you are just not naturally good at video games, just like some people are not good at sports or school and if that is the case you will have to put in a lot more hours to get up to a decent level.