Let's analyze situation:
- "constantly guarding his face" + "block mid punches" - it seems, that your opponent has good reaction and definitely would block single punches/middles.
- "can easily evade low-kicks" - that is a sign of good "leg performance" - your opponent is light on his legs, not grounded.
So, what we can assume:
- your opponent is more skilled, than you
- he/she is very light on his legs, not grounded
- he/she prefers distant combat
What you can do for complicating your opponent's live?
He/she prefers distant combat => cut the distance (that would also remove his/her bonus from being light on legs). Of course, that may be expensive, but that is the only way I see to change the situation. Try to cut angles, to make your opponent stand near the wall, preventing him from stepping back. Don't surrender/step back (it's important!) - make your opponent think, that he may break his knuckles hitting your head.
And then, after cutting the distance - go in exchange with series of punches. Changing levels, mixing with knees, elbows, clinching. Of course, you would definitely accept serious damage, but you have a chance to "outpunch" your opponent.
That is a costy way(so you should weigh the risks!), but as long as you are loosing in long-range strikes, I don't see any other.
This is not a silver-bullet in any way. Your opponent definitely may not allow you to close the distance, bring you a concussion with straigt punches (as long as you are approaching, these ones would be even harder, than normal) or knees. But you have a chance.
And also, while cutting the distance, you are gaining initiative. Judges usually like it.)
PS
Let's assume, that you've tried all, and not succeeded. Your opponent not allowing you to trap him in a corner, keeping you at a distance with straight punches/kicks, and you're finally tired. Then you should wait for a small handicap, a small slack. Catch it and use.
For example, your opponent may put down his left hand when doing a straight right (that's a common, usual mistake, which is rather widespread). Eat some such punches to make him sure in its dominance, and then throw a sudden right cross (in this particular example). Of course, you are likely to eat that straight right, but suddence you use may be decisive.
It's even more risky way - because, along with eating punches, you are loosing initiative and may be punished by judge for a passive fighting.
