But if you don't have all of that, keep going anyway. I had none of those things to start with, and the practice was still highly beneficial.
If contemplation of impermanence is causing you discombobulation, try something more concrete, like contemplation of the fact that you are inevitably going to die. If that's the kind of contemplation you mean when you're say you're having unwanted thoughts, not to worry. Those thoughts were actually always there, you just never noticed them before. The point of the practice is to develop the capacity to calmly abide whatever arises, including such thoughts, so the fact that they're coming up actually constitutes a good training opportunity. If you find you can't stay calm, start with contemplation of more remote deaths. Your own of old age, for instance, or the death of someone to whom you feel neutral.