Need help on improving CMD

I’m in a campaign right now. Lvl8. I have a character who has CMD26 base, 27 vs Disarm and Sunder, 30 vs Trip and Bullrush. We will Hit lvl9 soon and I will have a feat free. I don’t expect any gold or gear any time soon.

My issue is that the GM (who is a great storyteller) loves combat maneuvers. For example, I was put up against a foe with +26 trip. That means nearly always being knocked down. Before this I went up against a Disarm Master, which taught me the need for weapon cords.

Are there any feats I can take to help CMD? Besides for shield focus, all of my feats are geared towards offense right now.

Also the character is a Brawler (1) then fighter -> sentinel. So he gets 4x Martial flexibility

Shielded stand helps you get up when you're tripped, and you said you had shield focus.

If the enemies might use one of the movement combat maneuvers on you then flanking foil mitigates the effect of being flanked.

Ground grabber is +2 CMD against many effects if your enemy is larger than you.

Amateur swashbuckler (dodging panache) can add to your AC/CMD if you have a Cha bonus, and moves you away from the enemy possibly in the middle of a full attack.

Indomitable mountain style will take 2 feats (combat reflexes as well; I expect you have IUS) but gives you +4 CMD so long as you don't move.

Combat reflexes/bodyguard could add to others AC/CMD.

Thank you for the advice

How about Monkey Style? They disarm you? Punch 'em! They trip you? Fight prone!

Tripping, Repositioning, and Bull Rushing have Size Limits. If you are more than 2 Sizes bigger than your opponent, it's no dice for them. Increasing your Size in general will make you more resistant to Combat Maneuvers: +2/ Size Increase.

Freedom of Movement completely negates a lot of Combat Maneuvers, Especially Grappling, Probably Tripping, and some Dirty Tricks.

Ring of Protection protects you from Combat Maneuvers.

Increasing your Strength or Dex increases your CMD.

With 3 levels in Maneuver Master, you can make Attacks of Opportunity against opponents that try to do Combat Maneuvers on you even if they have Improved whatever.

If you take Improved (whatever), that increases your CMD by +2 for that Maneuver.

Shield Specialization is probably the biggest increase you can get from a single feat without having to make drastic changes.

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Other Feats you might consider
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*Solo Maneuvers
*Creature Focus[Ooze] ->Focused Ooze Expertise (This can be boosted further with 3 levels in Pathfinder Delver and anything that increases and/or grants trap sense.){eg. Improved Eldritch heritage[kobold]}
*Dodge -> Punishing Step

There are magic items that can help, but you said that those aren't realistic options for you. The above comes from a build I made that focused on raising CMD as high as possible.