Well, you've been feeling this way for a while, but I doubt any of us here will be able to convince you otherwise of your fear. You're afraid you have a brain tumor. You probably don't have one, even based on the evidence you provide, but I doubt very much that it makes much of a difference to you that I say that.
First off, that head rush feeling you get while straining is called the vasovagal response (don't google that!), and it's a very natural response to certain physical and emotional stimuli that effect that vagus nerve.
Also, speaking to doctors online is no substitute for the real thing. You're basing your belief that you have a brain tumor on the fact that you believe your symptoms are attributable to seizure activity, but you have ZERO evidence that that's the case. Anxiety can cause all of the things you're experiencing, and it's a FAR MORE COMMON problem than brain tumors, believe me.
My dad had a brain tumor, a fatal one, and his symptoms were nothing like yours. I won't get into particulars, but his were CONSTANT, SEVERE, and PROGRESSIVE.
You say money is tight right now. That's alright, it's keeping you from getting these tests that aren't going to give you lasting peace of mind. If you can't afford therapy or anxiety treatment, there are still plenty of FREE, helpful things you can do, which include staying off google, mindful meditation, deep breathing exercises, and general hobbies. I know this sucks right now—I know—but you have GOT to take even small steps to help yourself out of this situation. Right now, you're in a loop where your fear is perpetuating your symptoms, and your symptoms are making your stress worse, which breed more symptoms, and more fear. It's never ending if you let it be, but eventually you're going to have to short out that circuit.