Howdy doodle! Maybe I can help you out. 1) How do you apply your makeup and what form is it (stick, cream, liquid)? If you use a brush for example, and put foundation directly on the brush (if it was say, a liquid) then apply to your face, you could be applying too much. If that's the case, try applying foundation to the back of your hand or a small dish, and dab your tool (including fingers!) into that and work in thin layers. Apply foundation only where you think you need it to even out skin. That old saying 'less is more' can really help stave off cake face. And always blend blend blend blend bleeeeeeeennnnnddddd - if you're wondering if you've blended enough, blend some more (but gently please :D).
2) Hmm. I have the same issue with my nose. Does your nose get oily? Apply less foundation here and try using a puff or sponge to press in powder to the nose, but not too much otherwise it's more likely to get cakey. Makeup doesn't necessarily help with texture, just illusions of things.
3) What's your skin like? Overall dry, oily, combo, "normal"? Skin prep is important and I think very personal. If you saw dry parts peeking through, that area may need more moisturiser or to be lightly exfoliated. If you have combo skin, treat it in zones: e.g. dry areas get more moisturizer, maybe a heavy-ish one, and oily areas get a gel. As for dry areas that are sometimes oily...that's a tough one. Lifestyle could be influencing that, the environment..I've only experienced that in the transition from winter to spring/summer. sorry.
4) Patchy or separating foundation could be a number of things. Maybe skin prep/skin type and the foundation don't match e.g. dry skin not using enough cream with a matte foundation plus powder. It could be the foundation needs more blending with an appropriate tool e.g. stick foundations are usually pretty dense so a denser brush would probably blend it out better than a fluffy one, Or, a liquid foundation dries down really quick so it could be harder to blend with a brush and might be better with a damp sponge (happened to me with Fenty foundation). It could also be, and this one's annoying, skin care and foundation not mixing. Let's say your skin care is silicone-based but your foundation is water-based: the silicone will repel foundation and it'll look patchy (that happened to me when I tried Born this Way foundation over Clinique's Dramatically Different Gel). If your skincare and foundation are from the same family, I think a successful application is more likely.
Wowza, I really hope this is helpful for you. I want you to have a great foundation application experience and I can understand the frustration when you keep trying and are still like ARRRRGGGHHHHH
Look into YouTube videos by Lisa Eldridge or Angie Di Battista. Watching Lisa apply makeup to a variety of people helped me up my foundation game and Angie has some really helpful videos about skin prep as well as examples for applying to certain skin types. I'd also consider taking how beauty influencers (if you watch them) apply their makeup with a big grain of salt - they tend to all do it the same way e.g. 'baking' concealer after applying way too much of it. Keep trying, keep playing around for that golden combination. Good luck!