Okay, before I get into specifics, you need to know that there's a good news, bad news, good news deal going on.
The good news is that there are a lot of technical things you can work on to make yourself harder to pin, help you survive and feel more comfortable when you are pinned, and improve your chances of escaping when you are pinned - even against much larger opponents.
The bad news is that all these things are hard - especially against bigger, more experienced opponents. Someone can show you exactly what you need to do in every situation and it will still take a ton of practice and sparring until those movements become automatic under pressure. During that process, you will get squashed a lot. Think about it - if you have two physically identical opponents and one achieves a dominant top control position on the other. The top person now has a structural advantage and the bottom person will only get away if they manage to grapple significantly better than the top person at least long enough to effect an escape. Now imagine the bottom person is half the size of the top person. That means the bottom person is going to have to grapple
much more technically than the top person in order to escape. (There are some slight advantages the smaller person can sometimes have for escaping, but they are very much outweighed by the disadvantages.)
The remaining good news is twofold. First, once you learn what you should be working on to avoid getting squashed, the process is less frustrating. You may still get smashed consistently for the next year, but at least you know what mistakes you made and what you should be working on to fix them. Second, once you've been training long enough to survive and hold your own, you're going to be very technically skilled. Bigger people get away with doing techniques 60% correct. You're going to be doing them 98% correct.
Now on to specifics areas to work on...
First and foremost - never let yourself be flat. Always be on your side. In the instant that someone passes your guard, you should already be turning towards them to get on your side. If their pass is one that forces you away from them, be on your other side. (That requires extra know-how to keep them from getting on your back, but that's a whole area of study which is difficult to explain in text.) If your partner does manage to get you flat, work a mini hip bump and shrimp to get ever so slightly on your side facing the, If you can get even 2 or 3 degrees of rotation towards your partner instead of facing straight up, it will make it way, way easier to breathe. Position your legs as braces to make it hard for your partner to force you flat. Also do a crunch towards the side you are facing. That will make it more difficult for your partner to flatten you.
Second - do not give up underhooks. When your partner passes your guard, try gluing your top arm to your side, reaching your hand down to and along your leg. Squeeze your elbow to your side as tightly as you can. If your partner tries to flatten you out while you have the underhook, then they give you a lot more escape options. Use your bottom arm to hand fight and try to stop the crossface. Use your knees to block their passage to full mount. (There is a use case for the bottom person framing with stiff arms long enough to recover guard. I don't necessarily recommend that for a beginner working with much bigger, stronger opponents. It's too easy for them to convert those frames into levers to use against you or to take advantage of the opening to get underhooks on you.)
Third - realize that there are a lot of mini-battles between the guard pass and a fully secured pin. We have a tendency to see things in terms of static positions. Now my opponent is in my guard. Oops, he got past my legs. So now I'm in bottom of side control and I have to work my side control escapes. But in reality, there are a lot of little steps my opponent needs to accomplish in-between passing my legs and fully securing my upper body. If I can shut down any of those steps, I make it a lot harder for him to keep me controlled.
Fourth - the more technically correct your escapes from bottom are, the less strength they require. A lot of times when you try an escape and your partner feels just too heavy to move, it really means that your positioning or your timing was not quite right. The flip side of this is that the better the technique of the person on top is, the heavier they will feel. So if you're a white belt trying to escape the mount of a purple belt who outweighs you by 40 kg, it really sucks on both fronts. To avoid total frustration, don't fixate on whether you are able to actually escape in that situation. Instead, focus on two things. To begin with, see what you can do to adjust your position so that you feel more comfortable, can breathe more easily, and can defend submissions longer. Next, try to identify one technical flaw in your escape attempts that you can work on improving for next time.
When I teach class tonight, I'll try to cover some of what I just wrote and record some video so you can see what I'm talking about. You might also want to check out Priit Mihkelson on YouTube. He has a lot of good material on defensive structures from the bottom.
You can also check out this video I made on the basic concepts behind framing. It's not necessarily the primary area you should be working on right now, but it's an important aspect that you need to understand while working from the bottom.