I'm unable to attain strong erections, therefore penetration is quite difficult and nearly impossible without it.

Find the cause first: To assess if one's erections have weakened, he should visit a primary doctor, who can check for possible causes: stress, depression, diabetes, high blood pressure, low testosterone, artery disease, high cholesterol, smoking, older age, etc... Causes found can be treated to help make erections stronger, firmer, longer lasting. A urologist can be helpful. Meds like viagra, cialis, Levitra (vardenafil) can work.