So if inexperience is your problem, you just get some experience by doing the thing you want experience in. Like if you didn't know how to kiss at 14 years old, your high school girlfriend would've cut you some slack to figure it out. If you don't know how to kiss at 24, chances are the girl will be turned off and leave. But she'll leave you more experienced. So if experience is your main problem, it's a problem that's solved by trying the thing.
Because falling behind and missing developmental milestones are obstacles that will affect you and damage you mentally. But they're obstacles, and obstacles can be overcome. If you're so damaged that you can't overcome not having had a girlfriend in high school, then you're mentally ill.
If you're physically attractive enough to get a girlfriend (which you're saying you are if you think you're incel because your parents didn't push you) and you're not mentally ill, then even if you've fallen behind you can catch up enough to not be an incel.
To reiterate: if you're physically attractive enough to get a girlfriend but you're not mentally ill, then you don't belong here.