I'm only 20 years old and lately I've been having trouble keeping or even getting an erection during intercourse, what could be the cause of this?

Erectile dysfunction. Very common & there are many causes. Anxiety & other psychological causes likely most common in 20-year-olds. Alcohol, drugs & fatigue also common. Medical conditions can be neurologic (including pressure on nerves from bicycle seat), hormonal (including diabetes). Advise urologic consultation.