Scott, if you find altering your definition of masculine doesn't work and you absolutely must measure up to concepts drilled into you earlier in life, why not just pick a couple and do them? Maybe if you had just a couple activities that ward off this feeling you might be able to do them during insecure moments.

An easy and fun (and very manly) activity is to go to a gun range and shoot a gun. You don't have to buy a gun; they'll rent you one for a short time. It's an odd activity for me but I do this now and again and it feels very manly. Another bonus is that you don't have to be athletic to do it.

Another one that works for me is to create some angry music. Really aggressive stuff. For me this cultivates a "I don’t' give a ***** what you think" attitude while also feeling a sort of bad boy rebel vibe.

Paintball. This can be harder to arrange as you need other people but shooting others and getting shot is pretty butch.

Go camping. Not cushy resort camping but like no bathrooms, hike to your campsite type camping. I personally hate this but it works in the macho guy category.

Develop a friendship with a particularly masculine guy and do whatever he's doing. He wants to go play racquetball, join him. Who cares if you suck? The goal is to feel like one of the guys.

Change your own oil. Getting under a car, on purpose, is very male. Get some grease under the fingernails and then don't get them immediately manicured. This is another activity I can't stand but any automotive work I've attempted has made me feel very manly (despite ending in disaster and usually causing additional repairs I didn't intend).

So, I dunno, this is sort of a "if you can't beat 'em, join 'em" approach but a couple really manly activities may alter how you see yourself.

Cyran0