Emotional support-giving is a skill. Some people don't have it or don't get taught it, or even are discouraged from offering it. Most men are trained from birth not to feel their emotions, it creates a common condition called normative male alexithymia. Basically this means that possibly the majority of men don't even have the ability to name their own emotions, let alone empathise with someone else. Certainly I think it is rare for boys to be trained to emotionally support anyone, not in the way girls are in any case. (Generally speaking)
If your DP has low skills in giving emotional support, then trying to support you will quickly exhaust him. He is "unfit" just like someone who doesn't exercise much - trying to do the thing he has low skill in, will exhaust him very quickly.
Add to that, a person who has low emotional insight is likely to experience pretty much all negative feelings (worry, anger, guilt, sadness, exhaustion, pain, hopelessness, fear, etc.) as a blanket emotion of "irritated" or "angry", and all positive feelings as a blanket of "contented" or even "blank/no emotion".
This is important to know, because basically your DP may be feeling very fearful and hopeless (for example) about your MH - but if he already has low emotional insight, he may only express that as being annoyed or impatient or even angry with you. Add to that that he may be absolutely exhausted by trying to do something he is terrible at (i.e., offering emotional support), and it can make him very snappy and generally shit, sadly.
I appreciate how upsetting it is for you, but you also can't make a person be good at something just by requiring it of them. Maybe he is actually an arsehole, I don't know, or maybe he honestly just does not have the ability to support you in the way you want - either way - it's pointless getting upset with him over it. You will only magnify all the problems you're already experiencing.
If I were you I'd go to him for hugs, and then get on the phone for a cry with someone else. You will get through this time, even though your partner isn't doing a stellar job. Deep breaths and remember no-one can solve all the issues brought on by this COVID situation - it is very hard and something we need to essentially wait out.