I hear you. But also consider that if you are heading towards marriage or co-habiting with your s.o., then you won't get much time alone. You'll go to work and then come home and be with her again. Your only time alone will be when she has something to do which doesn't include you. I know that my hubby and I love the times when we're home alone and the kids are out too, but it is a rarity.
While I'm not saying you must spend all your free time with her, fast forward to when you're coming home every night to her. Think about whether it will be pleasure or pain....and then consider if the relationship is working for you.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.