Wanted to chime in as my husband is the same and it's so frustrating/sensitive topic! I'm exactly same as you, I'm always thinking we should do it more but try to remember it does only take one time! Especially if you time it right/get to know when you're ovulating. I use OPKs and do BBT.
Been TTC#2 for over a year now (on cycle 16!) but it didn't stop us having DC1 - that only took 5 months. But we were younger then, I'm 39 now. My hubby seems to have gotten worse as he's gotten older...
If only the wank and mount worked lol, that's not the issue with us - my hubby assures me he is mad for me (and I can tell!) so him going off and trying to, ahem, help himself wouldn't work for us.
We're still trying to work through this, some months are better than others, but my advice would be to try find out what specifically is stopping him. For my hubby, it isn't always just because he's trying to do it too many days in a row (although that's often the case).
We also discovered pace/position has a lot to do with it! Sorry if TMI, but he often finds he needs speed to get there in the end and was knackered! We have a toddler so we're often exhausted at night time, and he has a bad back... So we just worked out ways to get him there faster with less effort! Also switched to doing the deed during the day sometimes when we were both more awake!
I hope that helps or gives you food for thought, I'm so sorry and deeply sympathise with you. It's the part of TTC nobody talks about...