A story that might help...
When I studied jazz in college, it was new to me, and it was hard for me to solo because I had to think about what the chords were and where I was in the form.
20 years later, most of those tunes seem simple and I wouldn't think twice about soloing over "rhythm changes" or a "jazz blues" because I know those sets of changes and I no longer have to think about them. Other standards like "Girl From Ipanema," or "Satin Doll" fall into that category because I've now played them dozens, if not hundreds of times.
If you are new to the guitar, or new to the blues, the 12 bar form is not going to come naturally - yet. It will, but it will take some time.
1 - Listen to standard blues, a lot. If you tend to listen more to blues/rock or classic rock, the blues form won't be present as often.
2 - practice playing good old "blues in E" and count the measures as you go. The more you do it, the more you'll "just know."
3 - Play along with standard blues jam tracks, but don't solo, play chords for now.
The more you play chords along with the 12 bar blues, the more it will be something you "own" and you won't have to think about.