How will my lifestyle be impacted if I have lost a testicle or have a weakened testicle?

None at all. Loss or atrophy of a single testicle will not impair your ability to father children (assuming the other testicle is normal) nor will it affect erections. If the appearance of a lost or atrophied testicle is bothersome, speak with a urologist to find out what options are available for you.
Should not change. Not sure what you mean by a "weakened testicle". However, even in the setting of losing one testicle, the remaining testicle will presumably secrete sufficient testosterone hormone, and produce adequate sperm levels. Overall, you should not have any significant impact on your lifestyle.